There's an absurd gender dimorphism in most games where every guy is a mountain of muscles by default. I don't enjoy that body type - for myself, as a 5'5 dude, or even romantically - and women are usually on the softer, thinner side, so I tend to pick them at first.
If its a game where I can easily change genders, I'll flip around to my tastes, depending on which clothing looks better on whatnot. - Aliens:Fireteam Elite and Dragon Quest Builders 2 come to mind as examples that did that. Also games with intricate character creators, like Saint's Row (RIP) are welcome, but rare.
Eastern games tend to have softer men, so those tend to be exceptions. I recall picking male options in both Genshin and Path To Nowhere, and I usually enjoy the male leads in jRPGs.
I think this is my reason. I like lithe, acrobatic archetypes and will, for instance, usually prefer playing stealthy character classes when given the option. Guy bodies in games are (or at least used to be) blocky rectangles; they look like walking refrigerators. Gals usually have a more dynamic and nimble appearance.
Two more relevant reasons: (1) traditinally, non-customizable main characters are predominately male, so when given a choice I'll choose the less common option to mix it up and (2) I am a guy in real life and am bored enough of it that I feel incentivized to play the other side in game world.