It's almost as if they don't really want to discuss whatever the topic of their thread is; they just want to try their luck at getting merits.
If you were an account farmer and you were sorely missing a series of easy replies to something you've milked, how easy would it be to simply create an account or access a low-activity account, and create a thread to fulfill that purpose of being a megathread?
Some users simply continue the cycle of low-effort posts.