Because Universal likely sells a lot fewer Express Passes--as evidenced by the higher price and lower hotel access.
Slinky would be the poster child for on-demand Genie disaster. Every day the Genie allotment is gone by mid-morning. Without a regulator on
Genie+ arrival times, people would flock back there as soon as they get to the park and the Genie line could be an hour or more. You'd also be removing the limitations on the number of Genie passes that can be used in a day. The return times / 2 hour delay place some limits on how many you can use in a day. Without that, people would move from Genie line to Genie line, attempting to ride a dozen attractions or more with park hopping. I say "attempting" because again the Genie lines would be so overloaded that they would move just as slow as Standby does today.
That is, unless you want to impose crazy high prices and purchase limits.