The El Cortez does offer free parking for their guests in one of the attached parking garages. This is due entirely to space restriction issues.
Most hotels that have a casino offer parking for guests and visitors, but others only allow guests staying at their hotel access to their parking garage.
It’s actually more difficult to find parking you don’t have to pay for, but many locations offer parking that costs significantly less than paid parking on the Strip. One might think it would be easier to find hotels and businesses that offer free parking in Downtown Las Vegas than it is on the Strip, but that isn’t necessarily the case.