The benefit to a forum is that posts with new comments move to the top. If a Reddit/Lemmy post gets a single new comment it may or may not be seen again by anyone except the OP or of the comment was a reply then to the op of the replied comment.

Lemmy does have this actually

New Comments: Bumps posts to the top when they are created or receive a new reply, analogous to the sorting of traditional forums

https://join-lemmy.org/docs/users/03-votes-and-ranking.html

and then there’s the “Active” sort, which is kind of a compromise

Active (default): Calculates a rank based on the score and time of the latest comment, with decay over time