Not everyone has the same set of input requirements, I need 4 inputs for example. others don't have the gain in the source so need more in a pre. others are driving longer cables and most sources can't do that. a pre in a lot of systems sounds better then not having one even if you don't need the gain. volume control's are almost always better in a pre amp. Built in phono stages and DAC's. remote control. tone controls if you need them. etc, etc.
ok I think you get the point there are many reasons.