I think it's weird to consider it as a feature, they should offer two version like what this forum have, before we login, they ask how many minutes we want to stay or always stay logged in.
Yes, as for the most common site it's an option with the use of "remember me" that will take for several days mostly 30 days, this forum have that remember forever or something similar, but of course this are saved as browser cookies, if these cookies were deleted it will automatically logout the account. Most privacy-centered browsers deletes its cookies time to time for security and privacy reasons.