Since hybrid bikes may be a compromise with no benefits, why are they still often preferred for commuting instead of drop bar bikes?
Simply, I reject the premise of your question and I'm sure I'm not alone. I say that as someone who has a tourer and a hybrid, and commutes on both - the speed difference is much less than the variability in traffic volumes. 

In practical commuting conditions you're rarely aero-limited anyway, rather it's the speed of traffic, and to a lesser extent the acceleration. The extra visibility from the more upright position can mean getting away from a junction a touch sooner than in a lower position.

The biggest aero benefit from drop bars is when riding in the drops - and you don't often see people commuting in the drops. That position is much more useful for sustained effort than the typical stop-start commute.

Hybrids generally have a sufficiently forgiving fit that you can buy nominally the right size, get the saddle height right and go. No hassle, no need to be particularly flexible. Most commuters won't have heard of the concept of having your bike fitted properly, and would rightly have no interest in it.