Previously VRChat would remember the previous login and not require you to re-input credentials constantly when starting the game, but that seems to have changed. I think the most common use case is people using the same account on the same computer, and would rather not fumble with their keyboard (virtual or not) if they don't want to.
It's hard to tell if this was an intentional change or a bug, but either way I think it would be worth addressing for UX reasons, even if that means adding a "remember" checkbox.