Because these Mothers were never meant to be Mothers. They care more about themselves than they do anyone else, so even having children was to selfishly fulfill their own needs. They can't see past themselves. It's horrible and it does maximum damage to everyone but the Mom. 

Unfortunately, we live in a society that expects women to want to be mothers.  The world treats them like failures if they don’t have at least one child.  Look at women in the world who chose to be childless what are the reactions to that, the are called selfish, or unfeeling, or something must be wrong with them.  When if a man said “I don’t ever want to be a father.” He would be applauded for knowing his boundaries and for not taking on a role he doesn’t want to. 

Us Stepparents are damned if we do and damned if we dont.

Much like your BM, ours is the same.  Very selfish and married a very selfish man.  She did not do anything to support her kids with sports, etc because their SF said that was HIS time and made her choose between him and the kids. She chose him with everything.  Not in the healty "I need to put marriage first" kind of way.  

She too would only show up for photo ops or other mothers would send her photos and she would post them like she was there. My photos included. 

Both SS's cuts ties with her completely and its been 2.5 years of her getting drunk and texting/leaving messages begging them to talk to her.  She now has SF call and he has been warned one more call and they are going to the police for harrassment. 

This past mothers day, they spoiled me.  I told them it was not necessary, I am not their mother.  SS22 responded:  We know, but you never made us feel like an inconvenience.  Our mother just told us that SS26 was a mistake and she only had me to try to save her marriage to dad.  

So so sad.

I too felt like I was either having a breakdown or ready to break her face at times.  In my case, it ended where I no longer have to deal with her and have 2 SS's that are turning out to be better than I thought. 

Its hard not to feel bad, and empathise. She knows you love her and you show up.

Im a childless not by choice woman and it makes me sad that my skids parents both failed them.

Yeah unfortunately like mentioned above some women have no problem getting pregnant and birthing a kid but actually doing the work to raise the kid for 18+ years is just not in every woman's dna. 
 

Sometimes the idea of a cute cuddly baby is what some of these women are after but the reality kicks in the cuteness wears off and not every woman can handle the day to day grind.
 

If there was less stigma to adopt or give men (who are better custodial parents) full custody at the beginning then perhaps it might save a kid from a lot of emotional hurt.
 

The half a$$ed effort from trifling bio parents sometimes hurts more. 

For any parent to walk away from there child.  That there life is more important than anything else. Unfortunately this gets transferred to the kids.  Bad gene.   Kids at that age are looking for something, there not bags or adults.  Just a bad place in today society.  Want to. grow up, but love being a child. 
'Good Lucy, this normal works out in a few years 

I can never know what it feels like to be a mother, or a woman. I am am a man.  However, I do know what is to be a parent/dad.   

My parental feelings are due solely to my son.  When I realized that I wanted to make a life with my incredible bride, I knew I would have to be a father for her son.  She made me a dad.  I have never had a strong drive to be a father. Not then, not now, not ever.

Though I most definately am one.  I was all in as being SS-31's dad. I care very much about him. I love him.  

I cannot see myself feeling significantly different about a BK if I had experienced that. Though I will never know.  I could be entirely wrong about that.

No FOMO for me regarding fathering a BK of my own.

For me, what is precious is living well and living the love I have for my family.  No regrets on the what was misseds or what might have beens.  Not only no regrets, barely a thought about those things. Or no thougt at all.

Maybe that makes me less than.  I really don't know.

I do hope that everyone has the life they want and enjoy.  Those that do not and who are impacted by those who will not live their own best life, I hope the succubus or incubus entities detracting from your lives get their comeuppance.

All of the wasted time put into the why they do that is nothing but bullshit excuses. Shit is shit and shit people need to have that lable indelibly branded on their forheads for all to see for all eternity.  Metaphorically of course. I couldn't give a shit about the why of what they do.  What they do matters.

As for that kid pulling that shit, oh hell no.  I would go with....

 "Do not scream at me again you hell spawned turdlet.  We are going home, you will sit at the table, you will eat what you are served, and you will sit there until you eat every bite of it. I do not care what you want or don't want to eat.  I do not care how many hours, days, or weeks it takes for you to eat.  You will never again speak to me or anyone else that way again or you will rue the day you were born. Any questions?  Good. As for the rest of you, thank you for being polite, well behaved, and good young people. Unlike your ill behaved sister here."

Followed by loud driving while blasting angry screaming heavy metal at ear bursting levels.  Or, loudly played classical music.... all of the way home.

I understand you're hurt, I understand that this kid is struggling, however, tolerating, accomodating, or justifying this entirely unacceptable behavior is detrimental to you, to the quality kids in the picture, and even to this ill behaved kid on its way to shit spawn status.

With this one you are in a race to cut off her progression to being a shit herself, or her taking the path to become a diamond. Both are primarily the same material.  It is all carbon at some level.  Whether it takes the form of shit, or the form pf a D-Flawless diamond is up to the standards of behavior and standards of performance the kid is held to, and ultimately  to the choices the kid makes.

IMHO of course.

I suppose it is easy for me to be cut and dry considering that my world is generally pretty black and white. There is little grey in experiences or my consideration of things.  

I won the parent lottery.  Both of my parents are amazing people of character and quality. They are loving parents who raised their sons with standards of behavior and standards of performance and with zero acceptance of anything less than common decency from us.  

Which I suppose is the source of my cut and dry, black and white, no grey life/world views.

I have also ultimately won the spouse lottery, at least I won the second time I played, and the kid/Skid lottery.  

Though all of this takes work, commitment, collaboration, and clearly defined boundaries to create, nurture, and advance.

I get that my experience is far from common for many if not most people.  

It sears my soul that so many are living far more challenging experiences.

Find and live your best lives. Especially the kids.