They do this when they don’t want to have teenage kids and high school kids money left out of the box office, because if it’s called Scream 5, that’s all of a sudden a task for that demographic to catch up on 1-4. At least when they do this title, the generic Scream movie title, it feels like to teenagers they can come into it as a group setting without having context of the original 4.
Movie studios have used this tactic before with Halloween (2007) and other film franchises that have built off previous storylines. They don’t want to leave money on the table they could make.