The issue is transmission. On a roller coaster, people scream and yell. Well, that means the virus can be transmitted, and now to large groups of people very quickly. If one person in the front yells, they could potentially infect all the people behind them. Having 6 feet of distance doesn't mean much if you're all moving at 20 mph or more. Hence why a park in Japan initially told people not to scream on the rides (the now famous line, "Please scream inside your heart.") Don't think that would work very well here in the states.
Admittedly, most roller coasters are outdoors, so that may minimize things a bit--maybe even a lot, but who knows for sure? But even if they did, what about indoor coasters like Space Mountain? From a covid transmission point of view, that probably isn't very safe.
The other issue is all the rides in a park are totally different. Covid safety on an outdoor attraction is likely very different on an indoor attraction. And attractions with long indoor queues (think Indy or RotR)... also probably not very safe. Where do dark rides fall on the spectrum? The submarine ride? Star Tours? Water rides? Shows? Parades? Do you just close the indoor rides... and if so, are people going to visit a park if 2/3 of the attractions are closed for safety reasons?
Plus, we must think of the employees. I know I wouldn't want to run the elevator in Haunted Mansion. It's may be okay for the guest, who's only in it for a short time. But as an employee, being trapped in a fairly small indoor space for hours, with probably poor ventilation, crammed with people you don't know? I wouldn't want to do it.
I know I've proposed on my twitter that Disney ought to open more for food and shopping, but every time I have, I've been told the issue is unions.