Well I think its very balanced. And I think if both players are very good and equal in talent, Terran would beat Zerg for sure.

Zerg are most powerful because their main strategies of survival are pretty simple, they mass. If they lost units, its fine because they were relatively cheap. Of course thats not their only strategy, but in general, they tend to mass units.

The Protoss has extremely strong units, however they are very expensive so if you lose a few, your sometimes screwed.

And Terran are versatile, and need strong micro.

Out of those three, creating a bunch of Zergs are the easiest of them all to fight with, so everybody feel they are the strongest, because they can play better.

For example, if you let an above average noob to play with Terran on single player vs the computer they would lose. But if they use Zerg on single player vs Terran, they will win. It is simply much easier to use Zerg.