Committees are not good at designing effective presentations. You need one captain on the ship, while all others provide input rather than picking up the pen themselves.
Each person has a slightly different preference for structure and story lines. When there are multiple authors, nobody really understands what is already in the deck and what is not, resulting in duplication of people’s hobby horses on every slide. Each person has a slightly different style of writing.
That is why I am not a big believer in online collaboration systems where hundreds of people can edit a presentation Wiki-style (call me old fashioned). What we do need though is a sophisticated slide-by-slide commenting system to manage input.