Sometimes it is corporate policy, meaning if you are on a work computer you can't manually update it yourself.

There are still some legacy applications that require a specific version of IE, or specific IE functionality, in order to work correctly. The cost of "fixing" the application to work correctly may cost too much or be too much effort.

Of course, there may be others who may have no idea how to update, no doubt there is a whole group of (older) users where auto-update may be turned off and no idea the update process exists.