In regards to skinny people, children, etc. that people are bringing up,
Sorry, I can't come up with a better analogy.
I'm sure everybody is familiar with Subway, the restaurant.

Let's say a 6-inch sub is like a standard airline seat.
Lets say a bunch of people, skinny, average, and large, go for some subs and order 6-inch subs.

Children and really skinny people wouldn't be able to eat a fully loaded 6-inch.
That does not allow them to order a 5 or 4 inch sub. Those options aren't on the menu.

The average sized people might be able to eat the whole sub.

The obese people might be left hungry.
Is it fair if they go over to the other tables and start taking bites off other people's subs? Or should they order a footlong instead? They can't order a 7 or 8 inch, that is not on the menu.

People have mentioned this is not fair to little people or big people, etc.
Using the example above, United has a "Menu" of sorts.
1 Seat, use some or all.
If you are too big for one seat, you gotta get two.
No in-betweens.