Cleaning up an existing profile (regardless of whether it resulted from a merger) is a *lot* more work than entering a new profile. In order to be respectful of previous edits, one has to check each existing citation and understand how those citations relate to each other. That can be a lot of work, especially when the citations lack detail. When I clean up a merged profile, I sometimes just throw up my hands and move many of the previous citations into their own category, called "Other Sources".
So, you might ask, why don't you clean up the narrative? Well, the old citations only make sense in the context of an existing narrative. If I rewrite the old narrative, the old citations will make even less sense.
That's why I sometimes don't clean up immediately after a merge. But I try to go back and finish a job.
I'm not defending my habits, but the question was, "What keeps folks from doing post-merge cleanup?". Those are some of the good reasons why.