There is no one true replacement for the utility that it provides. Not with sufficient penetration of network effect, anyway.
Same reason most people don’t delete Facebook (edit: or Reddit).
You may not personally find utility in either of them. But just like boner pill spam, clearly someone does, or they would just go away.
The two are not mutually exclusive. But OP’s question was why “EVERYONE” has not deleted their account. And the objective answer is that some people obviously still use the service. But once again people are treating the downvote button as an “I don’t like this” button.
Edit to add: Why? What exactly is wrong with what I said?