Who are you benefitting by being a SAHM?
Yourself and your family.
Nobody else.
Ah, back to "there's no such thing as society".
100 years ago elderly people were taken care of by those not in paid employment, children were watched over by those not in paid employment and not roaming around in gangs.
By having a child who has attention paid to them, homework helped with, talents nurtured e.g. going to a sports lesson, music tuition. Etc. You are going to end up with a well balanced musician, artist, nurse, street sweeper.
If you have a child left home alone on YouTube and video games, crap ready meal parents out of the house at 7 back at 6 (yes they need to pay the bills and survive) you are not getting the same kind of child as the first.
I would prefer a society filled with the first kind of person rather than the second.
Young children require care. Male parents, female parent, paid employees. This doesn't stop because they are 6 or 9 or 11 or 15, you could even argue they need more care.
SAHP who actually parent, are benefitting society just as much as bus drivers or office administrators.