Pachinko would be legally gambling in the United States and the loophole they use in Japan doesnt exist here.
There are elements of Pachinko that make it unattractive for casino's to mess with. All the machines would be licensed tech from Japan (cost), managing the balls (if you actually distribute them out) has costs associated that they dont get as many scaling advantages that Japan would get, the machines go through more wear and tear than slot machines do. Also its my understanding you redeem the balls to get your prizes (though some just print out tickets now?), so you would have to create some kind of redemption mechanism for the balls (and also create anti cheating measures agains this).
It's just a bunch of work for what doesnt appear to be much gain. The main appeal to the game is that it has more illusion of skill than slots do. And the gambling authorities would have to figure out what the acceptable win/loss rations are on the machines.