It would be awesome if those who can show maybe a blood test from doctor showing immunity would be exempt.
There are a couple problems with that. First and foremost, we don't actually know that having the antibodies means you don't have and cannot spread the virus, nor does it mean you can't get it again. That's what happens sometimes with some viruses, but there are a number of cases where people appear to have gotten it twice. Did they never get rid of it and have just continued to have it, even though we thought they were "cured"? Did they catch a different strain we don't know of yet? Did they have a false-positive test at some point in there, or a false-negative one? No one is sure yet. So until people are sure, it would be reckless to assume that people could show a blood test and prove that they're immune. Second, there are a LOT of issues with test accuracy at this point, for both antibody- and active-virus-testing, particularly false negatives. Some estimates are up to about 45% of negatives are incorrect - that's horrifying! Until or unless we get a much better test, we can't responsibly say that someone showing they are negative or have had it already is not a risk to anyone else. Third, we're still not sure how long after a person had the virus, they stop shedding the virus. We have a guess based on some other diseases but we don't actually know. We don't know if a person can start re-shedding after a time when the virus "reactivates", the same way some people seem to be getting sick again or having long-lasting physical symptoms well after what we would expect to be the virus's duration. So how long ago did a person have it? What is the cut-off for when they shouldn't need to wear a mask anymore? And can people even get blood tests to prove it? Where I live, getting blood testing is virtually impossible unless you already had serious, hospitalization-worthy symptoms. Getting a nasal swab is easy, but that only tells me (maybe) if I currently have the virus; I can't find out if I've already had it.
And finally, and this to me is one of the more important but more controversial issues: if people see others not wearing masks, they will believe they don't need to wear one. Humans are social creatures and tend to model behavior after those we see around us. If people see that there is a rule and "no one is following it" (or even a large number of people aren't following it), they will come to the conclusion that the rule is unimportant and isn't being enforced. Even if the reason that those people are not wearing masks is that they have shown a blood test result meeting particular standards from within a particular time, if I walk into MK and see a whole bunch of people not wearing masks, I'm not putting mine on either until or unless I'm required to. And if a CM tells me I need to put mine on, my first response would be "but none of them are wearing them!" Either the rules need to apply to everyone, or they won't be followed by anyone. There can maybe be some exceptions, but they need to be clearly-defined, clearly-applied, and very limited in their scope so that other park-goers will not come to the conclusion that the rule doesn't matter.