I hate that they have to ask that too, but my understanding is the legal reasoning is to try and weed out money launderers or some shit like that.
As an aside, let me just encourage everyone here to use credit unions. I needed a very small loan once, only $500, to send to a sibling in another state who had lost his job and just needed short-term help. They wouldn’t approve the $500, but I was told they would approve $4000, which infuriated me as it was clearly a predatory tactic.
So I switched to a CU. My CU even taught me how to avoid overdraft fees in case I ever make a mistake. (Keep a line of credit open, that way if I accidentally overdraft the money would come from that and no fees.) I love banking with them.