There's a problem people are forgetting: There are two Lara Crofts.
(Well, technically three as there are three Tomb Raider continuities, but the second is heavily based on the first unlike the current one which is completely different)
The one(s) from the classic TR games, TR1 through Underworld, and the one from the reboot. And the two character versions could not be any more different from one another.
Anyone that played the game would realize Reboot Lara is out of the question, and explaining this would be like describing why Duke Nukem would never be considered by Nintendo for a guest Smash appearance.
But Classic Lara has similar problems. Her games (usually) weren't overly grimdark or stuffed full of horror elements, but she still primarily uses guns. The funny thing is I could actually imagine her moveset being almost exactly like Snake's as there are plenty of explosive weapons in TR, but the difference here is that Snake's guns aren't really an iconic trademark of his (don't think the M9 counts; not on the same level as Lara's dual pistols).
And as someone already brought up, she would conceptually be pretty similar to ZSS. So... there's the problem of not really bringing new things to the roster, in spite of Lara being one of the video game industry's icons. And her most major setback: This character is no longer in use. Crystal rebooted the series from the ground up and ditched this iteration of Lara Croft.
But besides all this... really, Lara isn't exactly a Square Enix character anyway. They only inherited the IP very recently, after absorbing Eidos.