Because of unrelenting high levels of stress. You can live like that some of the time, but not forever. I worked FT with one and while it was hard, it was doable. Throw another baby in the mix and the balls start dropping all over the place. Throw a third baby and you live your life permanently in the fast lane. May be fun for a short while, but it soon wears off. I started falling apart mentally and physically. Relationships can break down because there is never any downtime and no slack whatsoever, you are both tired and fed up. Kids live their lives in childcare or strapped in car seats while you are trying to catch up on everything.
Now I more or less SAH (self-employed very part-time), stress for everyone in the family has considerably decreased. DH can concentrate on his work and knows he won’t be bothered about anything home/children related. I don’t need to stress because I have the day to fit jobs in. I now can do jobs that never got to be done when I worked FT. I am in charge of my time and organise it how I wish. I have time and headspace to organise/clean the house, cook proper food food, get all shopping done etc etc etc
I only really have 5 productive hours while the kids are at school, so it is not ‘all day’ by any stretch of imagination. With a family of five, I am very busy, every day. But I don’t feel stressed, because I am in control and if I need a break I can make time for it.
Kids also got much better quality of life, they can have their sleep in the morning, go home after school and play outdoors or go to activities. I am there to enable all of this.
Everybody has nutritious meals and gets to live in a tidy ordered house with no stress.
Obviously, all the negatives of SAH which you listed in OP are true. But you reach a point where life is ridiculously stressful for everyone in the family and you are all miserable, but you get next to no financial reward for putting all of you through this sh£& for years and years and years. You pay out for childcare, for work clothes, travel expenses, holiday clubs etc etc What is left is just not worth all the stress and misery. Obviously we now have to manage on one wage, but we have gained in a lot of other aspects of our lives.
I have got three children and no family support though. Somebody who has got one DC and family support on tap may have a very different experience indeed and can’t see the issue.