Why is standing more common on hills than on flats? We all know that standing on a hill feels better than standing on a flat.

Even pros seemingly only stand up for hills (not counting sprints).

What causes this?

Here's what I've thought of so far:


Often you can ride up a hill without shifting out of the gear you were using before. If you were to stay seated, you would need to shift down.
For a long time (road) bikes had such high gears that you had to stand for many hills. That's not the case anymore though, so maybe it's a moot point.
When you're on a hill, the bike is tilted backwards. This makes it easier and more natural to stand up, because the bar is "higher" and more reachable than it would be otherwise.


But I have a feeling there's more to it than this. These reasons are not significant enough to explain the difference I can feel, or the way pros behave.
Because in the flats you reach higher speeds, and at those speeds aerodynamics become very important, so exposing more of your body to the wind is a drag.  The standing in flat is then limited to sprint bursts, or just used to give a little rest to your legs and buttock. Whereas going uphill is usually slow so the power advantage you may get standing is worth the trade-off for worse aerodynamics.