SlowSpyder
Lifer
Because it is more comfortable and loose clothing just catches on things including the wind. Plus bike clothing breathes better and wicks moisture away from your skin. It also dries out quickly when you stop.
I understand wanting proper attire for cycling, I just don't get the shirts with the sponsors and such. What you said can more or less apply to most sports/exercise. I see joggers all the time in running attire, but they don't dress like they're running in the Olympics with shirts that represent their country and a number on their back, that's all I'm getting at.