There's an abundance of psychological reasons that could be given as juxtaposition to your statement.
However the physical one's are more interesting:
1. Dog shits on carpet
2. Baby shits on carpet
3. Vibrator malfunctions, blows up and sets fire to carpet
Depending on who you chose, a fully grown male will USUALLY not **** on the carpet. Also, I am yet to whitness a male penis malfuctioning, exploding and setting fire to a carpet.