Because, Lara isn't played for humour as much as Nathan is. Nathan is a wise cracking, go-get 'em guy with a stupid haircut and insanely positive demeanor. He's much more cartoonish than Lara, who is put through uncomortable sexual situations in the game realistic violence. Of course, in gameplay, she's a fucking murder machine who ruthlessly kills men twice her size and doesn't flinch. Good job, devs.
If Nathan Drake was a girl, snarky Indiana Jones-type personality intact, then (s)he would probably still be played for slaptick laughs like she would if she was male. But Tomb Raider's tone is different; it's not a popcorn B-movie all around the world adventure story, it's some woman in a place she doesn't want to be and she needs to get out or she will die a gruesome QTE death.
Even so, Lara shrugs her various injuries off like any other action protagonist, and personally I thought seeing her getting killed and beaten up by nature, animals, wildlife, more animals, and cartoonishly scary men with guns hilarious in a black comedy way, which sucked a lot of seriousness out of the cutscenes.
So, Lara is basically like Nathan Drake, but with boobies, and in a much more serious story, yet she suffers the same intentionally cartoony characterisation, ridiculous injuries shrugged off because we wouldn't have a game if she died of cholera halfway through, and the writers really couldn't handle an Indiana Jones expy in a realistic story, but could you blame them? A lot was hanging on this game. Slip ups were made, implications were made, and we got a decent-ish game.
Besides, I don't think you've been listening.
[sup]From Kate Beaton's Hark! A Vagrant. Support artists, people. And reduce your carbon footprint.[/sup]