I don't know why, but it's just AGGRAVATING. Mostly because more than 50% of the time, the melodies can fall flat, especially if they're freestyled/not intricately arranged.
This video might be a bad example, but it's disgusting nonetheless
The first one (0:08), fourth one (5:01) and sixth one (7:32) agitate me the most.
This dude is over here belting it out over a pop-trap song.

This girl got in front of an AUDIENCE and not only made this song acoustic, but did a very half assed job singing it imo.
The last girl wasn't that bad, but I find it very cringy that she added a three part harmony to replace the spoken the "I want it, I got it" parts.
This song just doesn't NEED all of this

Giving a rap song a melody can translate over well with the right song. But this was just a mess

some rap songs (or songs with rap excerpts) should just remain rap songs