There are countless examples. They do carry them, and it may be required, especially during combat missions (and DCS is a combat simulator, right?). For MSFS, this might be strange, but for DCS, it's not.
However, I agree that it would be nice to have the option of wearing a helmet or not. That is, having the pilots' heads, without helmets, wearing only headsets.
Of course, this would probably require some changes (and contrary to appearances, modeling a human is one of the most difficult elements in 3D graphics combined with simulation), but it's possible. @ASC, maybe it's worth considering this option for users too?