It’s because helmets replace hair, and outside of some newer items Blizzard does not create different models for the two “beast” races. I think the first time we have seen different models for Tauren and Worgen was around Legion, but I might be mistaken…
Anyway, to take my Worgen Warrior as an example, this is the issue I think:
https://i.imgur.com/kP0Urxx.png
Pretty much everything up to the bangs, including the mane, is considered “body” by the game and not “hair”. You can just add bangs or braids down the side, but you cannot really change hair styles. Simply because what the game considers “hair” exists only in the front part of a Worgen’s head
This is a newer model that highlights the base problem. The gorget bit wraps around the poor girl’s head because that is where the game thinks the neck/shoulder is. Normal models have the hair as a removable object, but the mane for Tauren and Worgen is fixed in place, making helmet placement extremely awkward.
https://i.imgur.com/5yTFFmp.png
And an old model, to emphasise placement and “piercing” of the neck:
https://i.imgur.com/reemd7k.png
Unfortuantely, outside of completely re-doing how those models work and then remodeling all existing helmets since Legion there is no way to fix this issue I’m afraid.
For female models this is a bit easier IMO, you just need to lift the head slightly and pull it back a bit. Please excuse my poor image editing skills, but something like this: would already make a difference:
https://i.imgur.com/1ViIPbb.jpeg
This would allow them to classify the mane as “hair” and hide it whenever a helmet is put on, allowing for much more “realistic” helmet placement, and as a bonus would allow them to place and scale shoulder armour better.
Edit:
Obviously, especially for Worgen, the male models use a much more hunched over posture to give them a more feral, savage appearance, so straightening them up takes a lot more effort. And might also not be as welcome to the community. Personally, I think the male model - helmet issue notwithstanding - has always been better than the female one because it at least fully commits to the “beast” nature of Worgen, whereas the female only dips its toes (paws?) into it and still tries to retain some semblance of “cutesy” attractiveness. Kind of like female Trolls, where they are just Night Elves with weird feet and the male model got a distinct posture and identity. I’ll stop rambling for now 
