That's a very Basic/1e/2e viewpoint.
Once upon a time, humans were the dominant race in D&D because they were either the assumed race for a class (BECMI) or the only race that could advance without regard to level limit. In other words, you played an Elf, Dwarf, or Halfling because you wanted to play a very specific type of character, otherwise you played a human.
3e took away that motivation. Now you play a dwarf or an elf because you like dwarves and elves.
Fluff-wise, the base 10-15 years of D&D has actively promoted that opposite view. Humans are not the dominate race of the world, they're just one of a number of races that generally get along in the face of mounting evil.
I would imagine 90% of D&D tables would be upset if dwarves, elves, and halflings were not present in the game in some form. Probably 70% of tables would also be upset at the lack of half-orcs and gnomes. So multiple, human-like races are present.
Besides, being another race is fun. I have to wake up and go about every day as a human. Let me be an elf for four hours a week if I want to.