Hello Abigail it's been a while! Thanks for another marvelous post and now it's time to dig into it! Everytime i don't know where to start!
I think kids should learn what boundaries mean and are from the beginning like what's happening in the animal kingdom that you mentioned. If they learn from a young age that "you should be allowed to do this and not that'' and at the same time you earn something or lose they will end up getting the message. At the same time the parents even though i get that they may be tired from work, they must pay attention to their children needs and help the grow. It needs patience, it will be difficult but they need to lay the foundations from a young age.
Also about the low income families and much higher ones i think the key lies in spending more time with the child and not spoil him. Ofc that isn't absolute and it is just my personal opinion but from what i have seen rich families tend to provide too many things to their children, meaning too many unnecessary things so they child learns from a very young age to have everything that wants to so in other words no limits and boundaries are raised. At the same time the parents may not have the time to spend with their children a time that will be spend with a nanny, so no mother-father figure, but instead a person that has not full ''rights'' to manage the child as he/she wants too.
Poor income families on the other hand (except if they are the bad example and exploit their children :P) they will set boundaries right away because they have so little and they will even set responsibilities like for example the 7 year old to take care of the 4 year old. They might go to work and help their parents if it's let's say a farm or something so they will spend a huge amount of time with their parents.
Finally, to close it because again i read too much :P another thing is regularly see and it is a stupid ''reason'' yet logic in the parents mind of not setting boundaries is the cuteness and ''first time parents'' thing. They keep saying in their brains '' how can we punish a little kid'' ''i don't wanna be the bad father or mother'', ''ahh let him do what he/she wants, he/she will grow up and learn'' and like that they keep avoiding the problem. It's funny that in many cases that only happens with the first child that they don't exactly know how to manage it!