Because I'm old fashioned enough to think that children should respect their elders and betters. And giving up one's seat to an adult is a way of doing that.
OBVIOUSLY, there are exceptions for sick or infirm children etc, but we're talking about a general principle.
You are assuming, of course, that you can tell by looking whether the child whose seat you covet, or their parent whose lap you wish they would sit in, is capable of same.
If the seating is first come first served, then get their earlier.
Now, childrens towels flung over an empty seat so they can come and go while those who want to sit (of any age) cannot is not acceptable.
The adults in a pub equivalent, tablesfull of smokers coming back and forth, with half the seats empty at any time, while those who want to actually sit and stay in the seat are left standing around a too small bar height table, is just as selfish. I dont demand someone who is sitting to give up their seat, I do expect those who have a seat to actually sit on it!