I never said they were vector based. Most of them are cleaned up screen captures and graphics I cobbled together from different places.
I find it easier to move around and resize the objects in Illustrator -- create groups, fine tune positioning. You don't have to keep switching between layers, etc. The autotrace is fun if you do want to convert things to vectors.
But basically, I'm just more used to working in illustrator.
Personally, I don't see how people make complex composites in Photoshop -- seems like a lot more work.