Also a quirk due to Illinois law where gambling was once only allowed on working riverboats (that actually floated from dock and traversed the river for a bit). The laws were loosened to allow the boats to stay moored all the time, and they essentially became floating buildings, and, then the laws changed again to allow the Rosemont casino to be built as a regular building with none of the boat bullsh**.
City of Chicago has stood adamantly against casinos, but them (and the state) appear to be headed to allowing many more casinos to be built soon (rumored to be what "saves" the Bears and Soldier Field).