As this comment shows, even bad questions have some value in serving as a reference for, well, bad questions. However questions with a lot of downvotes tend to be deleted, so the system lacks a final bad-question-keep-for-reference state (besides open and closed).
the system lacks a final bad-question-keep-for-reference state
This is by design. Bad questions are only kept for longer if the answers are (or have been) somewhat useful. But even then, at some point, they will probably be deleted. I'd recommend you to read The Stack Overflow Question Lifecycle
If users see a lot of closed questions, they’ll note that we don’t enforce the guidelines, so why should they? Without any final resolution, asking questions that get closed becomes something we are implicitly encouraging — a broken windows problem. If this goes on for long enough, we’re no longer a community of programmers who ask and answer programming questions, we’re a community of random people discussing.. whatever. That’s toxic.
If enough of these closed questions are allowed to hang around, they become clutter that reduces the overall signal to noise ratio — which further reduces confidence in the system.