Because it is actually a very good, mature application server with lots of sane defaults and good, mature plugins, even if you aren't using a database. Basically, people are interested in using it for more complex applications than you are, because lots of its conventions are still quite good for those applications, even if it makes sense to reject or reconsider some other conventions that don't work quite as well.
> using it for more complex applications than you are
Are you sure? How complex do you mean? How complex do you presume the parent commenter's apps to be? Could you give an example of one of those more complex apps? I'm wondering if I'm currently building apps of the "simple" or "complex" variety, according to your terminology.
It actually wasn't my terminology... If you read the parent of my comment, the self-proclaimed complexity was, literally, "MVPs which are mostly CRUD apps". I think most Rails applications I've used defy that description, and I don't agree with that comment that it is "the entire reason for Rails existing" any more than I would agree that the entire reason for PHP existing is to make personal home pages.
Because it is actually a very good, mature application server with lots of sane defaults and good, mature plugins, even if you aren't using a database. Basically, people are interested in using it for more complex applications than you are, because lots of its conventions are still quite good for those applications, even if it makes sense to reject or reconsider some other conventions that don't work quite as well.