Also, I can't imagine people who end up sitting next to a kid whose parent isn't nearby, or the stewards who have to deal with such children would be happy about the arrangement either.
Although, I wonder if maybe the real reason for this might be to discourage families from flying with them....
In most companies, when you don’t book specific seats, the algorithm puts you alongside your children anyway.
Children should have privileges adults don't have. As they aren't full independent adults, it's fair to give them special treatment. Nobody is born an adult, so every person in society will get these benefits at some point of their lives.
At the same time, I don't see why airliners shouldn't be able to charge less for tickets that are truly "we will put you whatever we want irrespective of your needs and wants". Children who need to be by their parents shouldn't buy these tickets, the same way children shouldn't buy adult shoes.