Because this doesn't really work much of the time. The next bus is often nearly full. Waiting for a nearly empty bus might not happen for hours. There are many bus stops at WDW where there isn't a clear line - people practically shove each other to get on (many resorts, DTD).
As an example, we were almost first waiting one night: POR to MK. We waited over 30 min. for the bus (we had allowed 50 to get to our ADR). My MIL at about 95 pounds, had a sore hip. She was sitting on the bench to conserve energy. When the bus came, it was a mob scene (EMH night). If there hadn't been a group of us to help her, there's no way she would have gotten on, or probably the next bus either. Some of us ran from the bus stop at MK, and we were still late getting to Chef Mickey's.
Besides it's not like the ones who are there first really did anything special. It's not like people who wait for a show or a parade which has a specific start time. Being first in line at the bus stop is completely random. Sure they've waited longer, but usually only a few minutes.
I too, have seen people who really appear to be abusing the wheelchair system as desribed above (trading off). Luckily, they're relatively few, but WDW does run out of wheelchairs.
There are also plenty of people who feel that they have the right away over a stroller/wheel chair no matter what. It's almost funny - they'll cut you off to slow you down.
I am lucky I don't need a wheelchair, but the current systems does seem inefficient. I'm sure most people aren't annoyed with the person who needs the wheelchair, just the current system. It often takes more than 5 minutes to load one. Especially if the bus driver has trouble getting the ramp down properly. I'm thinking maybe they could add vans for those in wheelchairs at certain locations- if the sytem was good enough, it would mean less wait time for those in wheelchairs, and be less embarrassing. Maybe they could even have a system where you could call ahead from your hotel room - like they do with taxis, but maybe it would cost too much.