If you want long runs, yes, go west
But when asked the question about DH in Illinois, most people give the generic answer "there are no hills or mountains in Illinois". Which is true if you want to have miles and miles of lift assisted DH paradise....but we all know that wont happen in Illinois. But if you want to session parts of trails and hike a bike sections of trails for practice, you can do this in Illinois...you just have to be creative
There are sections of kettle (I know, not Illinois) that are fun and can be sessioned. Down in Ogelsby there is the DH course, and supposedly Mathiessen state park has some DH sections. There are others (that I wont talk about publicly
) There can be DH in Illinois, but the quantity is just not as much.