I can give you many reasons, but I will give you two.
1) You don’t need to be into fashion to recognize that wearing clothing is more than function. It’s also self-expression. It only takes a little bit of critical thinking and awareness of Jean’s history to know what kind of character she has always been. I refer you back to exhibit A where she pirouettes in front of Lorna which is proceeded by decades of going shopping with Storm, coming up with her own costumes, and has usually punctuated an outfit choice with what her mindset has been, barring maybe newer eras starting in X-Men. Jean rewrote her Marvel Girl out in DPS to die literally or figuratively as Marvel Girl, wearing the X-Factor uniform to go undercover as mutant saviors while also disguising themselves as mutant exterminators, Phoenix to signal her evolution back when she and Scott temporarily left the X-Men for Scott to heal, and the Revolution Era where Claremont wanted to sink in to everyone that Jean was Phoenix.
Big surprise: Jean subscribes to self-expression by what she wears.
Fun Fact: Jean Grey is the only Phoenix that has ever associated colors as phases of the Phoenix. No other Phoenix host does that. Only Jean! Even when Excalibur had a mini-panic attack at Rachel wearing Dark Phoenix colors, Rachel responded that it was out of pure preference. Meanwhile, Logan instantly recognizes for Jean that colors meant something in context to the Phoenix and to Jean, which is why she changed her outfit to Dark Phoenix colors as a signal to Logan in Endsong.
2) She is an Omega-Level Psychic. Why the hell would Jean worry about something she could physically do that her telekinesis can’t already do? Plus, have you not seen MatPat’s video on heels as a weapon? Those things are lethal.
Counterpoint: If there was a strict rule for how a character was supposed to dress, then there would be no variation in costumes. You seem to try to use the shoulder pads and headgear as if that the head gear actually protected Jean or that the shoulder pads would do anything for Jean.
I don’t even need to point to anything outside of superhero media to note how utterly ridiculous going for such strict ruling on costume designing is, as if a crucial part of Jean’s character was always function or necessity. Jean has never been that sort of character that went with function over self-expression, barring maybe the material it’s made out of, which even then is a bit of a moot point. Psylocke I can understand. Kitty I understand. In some cases, I can even understand Storm because an intrinsic part of her character is to hide certain tools from her thriving days. Jean Grey, a character so over-reliant on her powers that even when she loses them, she either runs or one-punches her foes is beyond me in ever understanding because it makes no sense for Jean’s character. I am not saying Jean is stupid by any means, but it’s never necessary for Jean and I can’t see Jean thinking in terms of what will happen when she can’t use her powers. Because why would she?!