Celebration hat question (and headgear)
This has been the mechanic since Guild Wars (One) days. It’s a suggestion that comes up, but has not been implemented. Whether due to technical reasons, or something else, is unknown. (I imagine having to create so many different ‘hats’ [one for each hairstyle] is resource-intensive.)
Good luck.
It’s both technical & design reasons — hair is animated (as well as some hats), and there are many styles. So ANet decided before the game launched that it would be better to offer more hats-creating-baldness (& also more hair styles) more often than it would be to offer fewer non-bald hats (and fewer hair styles).
Changing that original decision is costly: to have even one hat (e.g. celebration cap) show hair, they’d have to redesign the system and redo the artwork/animation for all existing hats/hairstyles to work with that new system.