For a long time rule one of monster movies was: avoid showing the monster as long as possible. The Universal films were a weird exception to this.
Put Mephisto on Netflix and you will never actually see him.
All you'd get is dark lighting, lens effects and Tony Todd's voice or somebody like that.
Think how much the Exorcist movies managed to scare people by doing very little effects wise.
Once you show a giant red dude in a loincloth onscreen he ceases to be scary/impressive no matter what eles you try to do with the makeup or effects.
Which is kind of the problem with Marvel's more monsterous villains. There's only so much you can do with a Kurse.
Marvel cant allot 15+ minutes of screen time to Kurse ripping peeps in half onscreen to solidify his scary rep. Not in a pg13 movie.
Besides, Marvel movies need to use that screen time to develop the love interest or the comical sidekick or both.