Since hybrid bikes may be a compromise with no benefits, why are they still often preferred for commuting instead of drop bar bikes? Since hybrid bikes may be a compromise with no benefits, why are they still often preferred for commuting instead of drop bar bikes?

According to at least one opinion, hybrids are a compromise with no benefits.
What is the downside to me purchasing a road bike instead of a hybrid?

Using drops bars instead can make you a few mph faster because the posture makes it more aerodynamic. I felt that my road bike with a pannier was faster than my hybrid bike without it! It seems like a lot of cycling commuters I see are missing out. My city's pretty bike friendly.

From what I've read, bikes with drop bars can be comfortable, be affordable, have wider tires, have fenders, and carry panniers.

Some of those features are more common in cyclocross and touring bikes than road bikes.

Interestingly, people find that drop bars make the bike more comfortable for longer distances. Often, when there's discomfort, bike fit or changing the posture or pedalling form is recommended. If the rider isn't used to such a low posture, the stem could be raised.

It's possible to get a used bike with a drop bar at a reasonable price. If a faster bike makes you drive less, you might save more even if you had to pay more for a faster bike and its parts.

It's possible to get bikes that have eyelets for mounting racks for panniers, and fenders. If they don't have them, it's possible to get adapters so that they could be mounted. Another option is bikepacking bags.

So far, the only advantages of hybrid bikes that I know of are peripheral vision and braking power. In bike friendly cities, they shouldn't be as much of an issue.
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.