Maybe because they are smart. US military pilots and aircrew are prohibited to tuck their pant legs into their boots nor do they wear their gloves over their sleeves. Basically to do so would make them funnels letting flammable liquids to your body in the case of an emergency situation involving fire.
But mostly that is the real reason.
Its more of a rule and not a prohibition when you actually get on the flightline. But also A: the boots worn by a US pilot are not the same as the boots worn by an xwing pilot. Tucking in a flightsuit to a US boot is not a comfortable thing, or a practical thing. I havent done it with an xwing suit yet, but i cannot imagine its as bad as if you tried it with a US uniform. I have seen the boots bloused and i have seen where when the gsuit is worn or any other leg mods like kneepads or kneeboards, it can cause the suit to gather enough to make it look like your suit is tucked when its just shorter on the leg.
B: US boots are much shorter and tighter as well. (at least comparing to Luke or some others) With zippers to the ankle cuffs its just not practical. The zipper pull would dig into you ankle over time and not be comfortable at all. Its not a small zipper. Plus sitting in the seat causes the suit to ride up even further. so generally the suit is fit a bit long to accomodate for all those variables. Tucking in your suit would just pull out as soon as you sat in the seat. I noticed with Poe, he had his untucked as well.. so i figure its just a preference thing. Maybe Luke was used to tucking his, like he did with his pants on Tatooine, and the boot wraps, so he did it with his flight suit as well...
C: the US pilot flightware is much more form fitting. Gloves especially are skin tight for the tactile touch and, like you said, fire retardant nature... the cloth portion of the glove anyways. I would think the leather part has a flame retardant coating as well, or even a permeation of something into the leather itself. However, having worn those for years and killing a few pair of those myself with heat or getting them wet, i didnt physically notice anything out of the ordinary about the leather outside the color. Not that i really paid attention to that. Even wearing those in the summer time isnt too uncomfortable, but we would regularly roll the sleeves of the flightsuit and the glove gauntlets to get some air, against what would be the rule, but it was more common than you might think. Also there are some who tuck the glove in the suit, some who tuck the suit in the glove, and some who just bunch them together or roll the glove against the suit cuff.. all kinds just for preference really.
At any rate, it had to be directed for the film. And it was someones job to make sure they all had the same stuff. I think the other interesting thing is that outside of the "character rolls" all the xwing flight helmets were adornment free except for the rebel symbol. Clean white and a nice blue symbol. It was impractical for them to paint all those helmets for 5 seconds of background screen time. The cool thing is that untucking the boot is actually canon because of that scene. if you want to take it that far. Which some do. For myself, i just say make it and wear it and who cares that it isnt exactly like Luke or Wedge. At some point it will happen on screen and then you can say you knew it all along.
Incidentally, i finished my belt last night... waiting to finish the harness... ran out of webbing. Ordered more. Also got the fabric for the boot cuff so i will start that next while i finish the first draft of the vest. I looked around to find some white nylon of the correct weight but have not ordered that yet.. I also got the data cylinders in the mail.. and will make the belt and boot flares when the soft parts are done. I found the right diameters of dowel rod so that will come soon. Then i will start looking for boots and gloves.
