Are there any other reasons you mentioned? I only found this one.
Thinking about it, people replying to the post stake their reputation too in whatever they post. If we see a throwaway or newish account causing shenanigans, people interested in the job post can form their own opinion of the nature of the comment? (isn't that what downvoting is for too?).
Since threads have this nice toggle feature you can always click [-] to ignore whatever people is saying about a job posting.