In addition to what everybody else said about fandom influences and whatnot,
It's pretty easy to find those shirts in smaller sizes (even if they have to shop in the kids' Sunday clothes section), when ironed the fabric tends to have a little bit of stiffness/structure to it so they're not form-fitting (which helps with residual binder bulge and conceals their narrower waist) and they can make their shoulders look slightly wider, and finally bowties add volume to the neck area and draw the eye away from the fact they have no Adam's apple.
In the end, that's just a completely nonthreatening look. MTFs will try to
aggressively flaunt and project their "femininity" upon everybody around them every waking moment, but every single FTM I've ever met in real life, no matter how loud or debauched online, either consciously or subconsciously tries pretty hard not to stand out
too much. I don't know if it's female socialization (
@Accept Only Substitutes might pitch in here?) or what, but while MTF displays seem to be all about maximizing perceived "femininity" and being called a True and Honest Woman, FTM displays go more towards minimizing their own femininity and trying to be called anything
except a woman.