when i make custom headmodels, for male at least, i use male 09 as a base making sure the eye and nose level are even so at least glasses fit without distorting the face, hats are a different story seeing as thats when head shape and hair come into play.
if we were to edit all models to be able to use a larger majority of glasses, hats and have necks fit suits, it would require us putting the model into blender, attaching it onto a skeleton with both glasses and hat bones, fixing eye level, fixing headshape to the best we can without distorting it, slim the neck down to fit similarly to male 09, add flex controllers to stop infinite eye rotation, and then compile everything too, not to mention if i were the one doing this then i'd also go and do the same for all female models.
while the list seems short through text, the process is much more time consuming than you might think with the amount of headmodels there are accessible not including custom models. while yes, it could be possible to do, it brings the question of who currently in the modelling team would be willing to do it and has the time to do it all. take everything i say with a grain of salt im still new to modelling and im still learning, if im wrong about anything anzati can correct me