1. It’s not financially viable
There’s no way of controlling a special section’s inventory: in other words, some flights are full of families, others are not. And airlines can’t afford to hold seats for a specific group of people who may or may not book them.
2. Some people might get offended and sue
Crazy, we know, but there are likely people out there who believe getting caged in the back of the plane just because they have children is kinda discriminatory, or at the very least, take offense to being seated by the bathrooms. And you know the family section will be in the back, because: a) people in first class won't want to be anywhere near a kid section, and b) it basically already is.