Why are the vex simulating the past?

Because they can travel through time.

It can't change. They should only be simulating the present and future.

Honestly, the problem with the Vex is that they don't make sense from a narrative standpoint. For instance, if they can't simulate what we will do, without getting into a whole Light/Dark thing, it makes their whole system moot. Not to mention, on a cosmic scale, being able to ensure things go exactly as planned are really unlikely and nearly impossible to ensure.

Like, I could live an extra day, buy a lotto ticket, this throws off the chain where XYZ was suppose to win ABC and that money was used to get DFE through school where they ultimately learn the cure for cancer. So they have to ensure I die and I die in a way that won't throw off something else, like maybe I get ran over, now the guy who would've helped create someone that solved world hunger is in jail and they never come to be.

Going back to the narrative point, I think Bungie just does a lot of cool time stuff with the Vex without really thinking about how these mechanics would come into play. Like, it creates a paradox to go back in time and change anything that happened, since there is no longer a future where I travel back in time (it's possible if we jump to another reality though). The chamber also sounds really cool, it's just impractical (too many ifs), doesn't seem to work flawlessly, making it useless, and the Vex's overall goal is too simple to need such a thing.

Like, they could easily put an end to us, but they don't, so we must do something that they need to happen in the future, yet why can't they just go back and end life before it developed? Heck, why does showing Osiris there is a future where they don't win a big deal. There are infinite of them, there had to be one where they don't win or at least that one robot dies.