In most games the choice of modeling human targets is usually dictated by the age ratings the company wants to achieve. Most flight sims have a "mild violence" warning, which would probably change if we could have visible human models getting killed in high detail.
The fact that in a flight sim we also have "nastier" weapons at our disposal is also another consideration. For example, dropping napalm or strafing infantry with cannon fire would be a complete gore fest and if the software publisher wanted to avoid adult type ratings, they would have to go for very basic animations in such a case.
I don't know exactly what's the deal with the current flight sim series and where the developers stand on this. However, it is possible to have visible crew on ground vehicles by editing a line in the conf.ini file and you can get them killed, the animations are basic and not really graphic. I also remember seeing infantry scattering when i was strafing trucks in the old IL2 series, but i think that feature was omitted in subsequent releases.
As you can see, generally speaking it's a matter of how much a software company is willing to be restricted by the ratings system. First person games like call of duty or even more tactical ones like ArmA of course feature more detailed animations on human models because that kind of combat is the main focus of the game, but they usually also sell more than flight sims so it kind of evens out for them.
Flight sim developers have to take into account the fact that maybe they would be cutting into their limited slice of the gamer demographic by including such content, while at the same time devoting resources to stuff that isn't very visible to us players when we zoom by at 300 mph.
Personally, i don't really mind much either way. If the sim starts branching out to ground combat then of course it will need some extra detail in terms of foot soldiers. Otherwise, seeing blood on canopies or when strafing troops, etc is not something i object to in the sim but neither is it something i consider a top priority.
The thing is not to go overboard in any case. For example, i wouldn't like to have ridiculous situations where the sim turns into a splatter movie, just like i wouldn't want to see a little angel sprite evaporating from the enemies i "kill"
