I think it was part of the story that some group had already broken into the tomb but got wiped out by the traps. You're looting their equipment. You even fight them in between puzzles.
In most of the games she’s racing against another group, but several times is alongside them or even ahead of them. For example, in the very first game she’s working with someone else who sends her in first to retrieve the treasure, and then steals it from her.
Plus, a good chunk of the time the items are hidden inside priceless ancient artifacts, and Croft has to dig into her British roots to destroy an ancient culture’s legacy so she can get a few bullets or whatever.