A lot of people will simply never learn the rules, even the Critical Role actors after years of playing professionally in front of an audience need to ask Matt Mercer for rules reminders...about their own PC's abilities.
If anything, I'd say ita testament to 5E that the game can function well even when players have no idea how to play their character.
So, this is something that I can attest to as well. For, example, my spouse, who is a highly educated upper level professional, has been playing in my home campaign for years, and understands the rules well enough to play their character, but doesn't much know or care how they apply to other classes, and so on. They need reminding all the time, sometimes even which die is which.
Edit: Critical Role is a wonderful example. Those guys have thousands of hours of experience and are talented, successful professionals, but only Matt Mercer has a true "DM mind." Liam O'Brien and Taliesen Jaffe come close, and the rest are various degrees of hopeless when it comes to memorizing rules, or sometimes even basic math. Yet they are all
fantastic D&D players that any DM would love to have in their campaign.
I think those of us who are drawn to DMing mostly have minds that are very copacetic with grokking complicated game rules, and so we don't understand that it is a fairly specialized kind of intelligence, as well as a fairly unusual interest.
One thing I have learned after decades of teaching is that every mind is unique, and folks can been brilliant in some areas and not so much in others. For example, I spent lunch yesterday working with a student who simply could not wrap her head around certain aspects of conventional argument structure, but when I shifted to a verbal question and response format suddenly relaxed and was able to express herself at length and with great eloquence.
One thing we have to be careful about as teachers, and often struggle with, is projecting our own model of the mind onto others. I think it is very much the same for DMs. Just as no two students are exactly the same, neither are any two players, so we have to have different strategies in our pockets and be patient and flexible. I ran a D&D summer camp for neurodivergent kids, and that was a huge learning experience.
Edit: I also think the age at which we start playing is significantly important, but I don't have research to back that up or anything. Anecdotally, a high proportion of DMs seem to have started playing very young and stuck with it, and I wonder if we sort of embedded certain aspects of game comprehension at a time when our brains were more plastic.