The most straightforward reason is that if you need a transactional database anyway, then moving the queue into the DB allows you to atomically en/dequeue messages at the same time as making other updates. Which can massively simplify your architecture because it eliminates an enormous category of possible failure modes. (Or it can massively improve your system correctness, if you didn't realize those failure modes were possible.)

Also most of the times you already have a database so why not use that instead of adding another service to the pile.

Using Kafka as a work queue is widely documented as a mistake. Using a single database results in a lot of operational simplicity and you get to skip a lot of distributed systems when all your state is in a single system

Because that’s sane, easy, and boring.