Why do bus drivers let lots of people stand near the doorway when people want to get off?

The bus driver has no way of knowing who plans to get off at the next stop. It is logical that people who are planning to get off will stand near the door so that they can easily do so. If someone who actually isn't planning to get off also chooses to stand near the door and to get in the way of people who are trying to get off, that person looks just the same as someone who is planning to get off. You can't tell.

Also bear in mind that just driving a bus is a complicated activity, which involves watching the other traffic closely to avoid collision, and so forth. A bus driver has only so much time and attention available to direct the passengers.