It doesn’t need to answer the question of how things began any more than our own understanding of our world answers that. The “Big Bang” is just the start of the simulation.
And I think you’re wrong to disagree about the purpose of our existence because the entire point of a simulation is to get information and data about the “real” world by running the options in a simulation. If we are indeed in a simulation, then the purpose is to give the creators of the simulation more information about their own world.
Ironically enough, it would also infer that these beings created us in their own image. Otherwise, it wouldn’t really be useful to them.