If there is one resource the Imperium has in abundance, its manpower. There are untold billions on each hive planet and the Imperial Guard is beyond counting. So why bother creating servitors when manpower is so cheap. You need more strength, put more people on it. Is the enviroment too deadly for humans? Who cares as there's always more where they came from and the Emperor wants you to suffer.
In fact, I'd be hard pressed to think of any job servitors can do which regular humans could not. Not in the callous Imperium where human life has very little value, jobs are repetitive by nature and mostly inherited and indentured servants, penal workers and penitant workers are easily acquired.
So why 'waste' valuable technological resources on building & maintaining them?