If a wrestler goes for a leg drop but his opponent rolls out of the way, then the guy doing the leg drop will sell falling on his ass like it really hurt him. But why? His ass would have hit the mat hard even if he'd hit the move, right? Same kind of thing when a wrestler goes for a drop kick and misses; either way, he'd be jumping up into the air and falling back down with nothing to break the fall.
You're right. But at least in the leg drop situation, it likely is accepted that landing on a person cushions your fall. And in some cases it does work.
FWIW, the more likely injury from a missed drop-kick is strained muscles from over-shooting, but that's harder to sell.
A lot of old school shows used to feature wrestlers throwing "fireballs". Jerry "The King" Lawler in particular was fond of them. These were actually just bits of flash paper that looked cool but didn't burn very hot. But what were they meant to be in kayfabe? Functional Magic? Ki Attacks? I get why wrestlers with supernatural gimmicks like Undertaker or the Great Muta could sort of get away with this stuff, but did the old Memphis crowds actually think Jerry Lawler was pyrokinetic or something?