A lot of closed questions do get deleted automatically. They have a few metrics to decide which questions to keep: https://stackoverflow.com/help/roomba

The same "question" can be asked many different ways, so it is useful to close all but one duplicate and have them all link to the authoritative Question.

This allows for Googlers to use several different wordings for a question and still find the single page with the answer.

Money.

The number of times I've received a quick answer from a closed question (as in marked off topic or something other than solved) is relatively high. Moderators tend to be either too heavy handed and nepotic or completely hands off. It's not an easy job but, seems like entrenched interests have always been more important than anything.