1. We (humans) are the only intelligent lifeform that we know of.
2. Roleplaying (acting) is difficult enough to do when you are simply RPing a complicated person that is not yourself.
3. Other races in D&D (and other games) tend to consist of two things:
a. A collection of mechanical bonuses/abilities; and
b. A bunch of stereotypes, either written as lore or generally known, that you play into.
4. As such, people generally choose races for their mechanical benefits, and then roleplay those races horribly as some sort of exaggerated human. "Oh look. It's a dwarf. Guess we'll see a pseudo-Scottish accent, and love of beer and/or rocks."
5. Given that people have difficulty RPing humans (which at least they have a wealth of models for), I would suggest that most people try to RP a human well before they branch out into terrible caricatures.
....better?
No.