1). You are a cheater.
2). You have no skill whatsoever.
3). You are terrible.
4). Why are you still reading this?
Reasons senties are unbalanced:
1). Automatic quick aiming
2). HIGH rate of fire.
3). The ONLY counters for it require you to have a Medic, who is not always available.
4). If you are LUCKY you MIGHT be able to drop enough grenades to blow up the sentry before it kills you.
5). Even if you DO have a Medic, if there's more than one sentry, you might as well hit your +explode bind.
6). FOUR ROCKETS AT A TIME, AND THE ONLY COUNTER IS AN AIRBLAST.
There are more, but just so you know. Sentries are unbalanced, horrible, cheap ways to play. And if you use them, you make Phlog Pyros look like upstanding citizens.