Because someone commenting leads to others spending time and effort responding. Deleting the comment breaks the chain. Don't comment if you feel that it's something you might want to delete. Think of commenting as like sending an email, but you get a short window to delete in this place.

This comment [0] from dang might be relevant:

> In case it's of interest, here's the standard language from emails I send people:

> We try not to delete posts that got replies, because doing so would be unfair to the other commenters in the thread. What I've done so far is reassign it to a random user ID, so it's as if you'd used a throwaway account to post it and there's no link to your main account. Does that work?

[0]: https://news.ycombinator.com/item?id=40734348