Why did Blizzard make Zerg so over powered?

Alright all of my fellow Starcraft 2 players, this is no secret, why is Blizzard so intent on making Zerg so OP? I mean I'm a bronze league Terran and every time I go online to play on my ladder, I lose to Zerg's zerglings, banelings, roaches, and most of all mutalisks. Every strategy I go against these zerg players causes me to lose. Strategy 1, mass marines is countered by banelings. Strategy 2, tank marine is countered by zerglings, banelings, and mutalisks. Strategy 3, Thors lose to mutas (although the game says thors counter mutas.) Then if I don't act fast enough I get 6 pooled. My fellow protoss friends agree in this belief because speedlings always destroy them. So, I conducted an experiment of playing 10 ladder games against each race. First, I play against Terran opponents and win 7 of 10. Then I play 10 against protoss which I win 8 of them. But then I face the Zerg players and only win 2. So why does Blizzard make Zerg, so overpowered?