What do you mean there is no cost? Aisle and window seats are more valuable and can be sold for more, and this would force airlines to sell them to families without any up charge they would've received from other customers
Most people don't give a shit where they sit, so most seats are not reserved. Traditionally, airlines tried to just put people close together when they booked together. When we check in, we just get random seats that are close together. That's okay. I'm fine with taking whatever seats no-one else wants.
If I understand United marketing correctly, they will actively sit you apart from others in your group unless you buy an upgrade. That is, instead of assigning you some of the free spots close together, you get put as far apart as possible, and they hope that you will buy an upgrade to sit close together.
Other airlines don't do that.
I am very tall and I always pay for a seat with extra legroom in economy. Whenever I’m picking my seat early, almost every seat in economy is available. People could pay to reserve a window or aisle seat, but anecdotally it seems like almost no one does this. Everyone I know just tries to check in as early as possible so they can grab a good seat before they’re all taken.
I don’t think airlines are actually losing any money by seating families together. It’s not like all those window and aisle seats would have been paid for otherwise.