Only SNL writers know how to write (well) for SNL. It’s a very specific style that takes time to learn.

The host’s monologue is the crown jewel of every episode; they won’t trust anyone else with it. Making sure the opening of the show is funny and hooks people is the most important part of the show. Stats and ratings prove, if the monologue sucks, people will change the channel. Even among the SNL writers, only the best of the best are trusted to write or approve material for the monologue.