Here is why zerg is the worst race:
1) Starter unit zergalings can't shoot and are weaker than your opponents workers.
Terran get marine right away they can provide air defense or ground defense, and kill
zerglings even before they can get close.
2) Zerg can only build on creep. This sucks, terran can build where ever they want to and they don't have to worry about creep.
3) Zerg loose workers when to bulid buildings.
4) No way to repair units, except roach, terran can repair units with SCV's.
5) Zerg need to constantly keep clicking queen to spawn larvae.
2 is not even a factor. Protoss can only build near pylons, which is the same as the creep.
For 1 - zerglings are useful in the very early stages of the game and when used mixed in with large numbers of other units. They are not meant to be an overwhelming force.
The Zerg strategy usually revolves around 1) Controlling the map or two 2) killing VERY quickly. The Zerg do have deficiencies, but not for most of the reasons you list. Number 5 is probably the biggest issue.