Why are 1/3 to 1/2 of HN submissions labeled [dead]?

Users can flag posts on the "new" page. Users flag things that look like ads, things that are in foreign languages, things that are duplicates, things that are blatantly political, and things that seem like outside HN's scope. (On the last one, for example, discussing the technical aspects of Billie Eilish's music is at least somewhat within HN's scope; a Billie Eilish fan page is not.)

Rarely that high; i see less than 1/7th of the "new" page right now marked [dead]"

Often its bad / dupe links, poor titles, etc. "Self promotion" often has a harder time getting attention.