I thought the same thing, just get a larger one to go over the inner skylight sticking up.
Two reasons that didn't work.
1st - The hole is close to the side of the roof. Not a lot of room to go bigger.
2nd - The inner skylight sticks up through the hole. It is square in shape with square corners.( see picture). This presents a problem. The after market skylights are more of a bubble shape and would have to be significantly larger to clear the square corner of the old inner skylight. Believe me, I researched it. Once I found out Fleetwood still sells them direct it was a no brainier to buy.