While I agree with this, the hackers have an incentive to get in and out as soon as possible (at least, with accounts that have valuable usernames), because they want to swap the username over to an account they fully control before the rightful owner takes the account back. While DMs were read during this exploit in some cases (I've seen this be the case for several musicians), valuable usernames were likely signed into, swapped, and then signed out of. That's how rare username theft on Instagram generally works, anyways.