That has various reasons:
For the Infantry it is the reason of getting rid of the fact, that Some Units use the same Hats as others (Like Simbirsky Officer and the Rifles Officer etc.)
/ so that the horsehair would actually move like the French Cuirassier Helmets otherwise hair-plume-thingy would be stiff and just look like its a log attached to
a cuirassier helmet that got some hair glued on. (Same goes for the nr.1 chevaulegers)
(thats also why there is the possibility for the Landbataillon Stuttgart to be hat/helmetless cause otherwise they would use the drummer shako
(which would be the musician helmet and a Landbataillon had only access to Pikes and yellow Armbands.)
The Helmets of the Mounted Jägers however are visible in 1st person because it was at the time faster to put those helmets together
(since I already mounted those on the "head" bone.) (and to be honest I have no Idea how it would be more benefical to play in 1st person for a Hussar,
So after I realised I actually didn't need to I just thought: "well... fuck it, I keep it that way."*)
And those were the various reasons.
* I actually thought in German though