If you spend more time closing issues than creating them manually from discussions, the math adds up.
As a maintainers, if you want to be be able to tell real issues from non-issue discussions, you still gave to read them (triage). That's what's taking time.
I don't see how transforming a discussion into an issue is less effort than the other way around. Both are a click.
Github's issues and discussions seem the same feature to me (almost identical UI with different naming).
The only potential benefit I can see is that discussions have a top-level upvote count.