I don't like this speculation about what an individual cannot or cannot learn to do, but I'd like to point out that Max Park wasn't always as dexterous as he is today. He was clearly able to learn to become extremely dexterous, both with one hand and two. You just don't become #1 in the world at OH with only CFOP and lookahead. Dexterity and coordination (after a lot of practice) are still significant factors to reach that level of world-class consistency.
I'm sure there are numerous cubers who feel they're better at repeated single flicks vs double flicks and therefore never intend to focus on improving double flicks, so they don't use them. Personally, I find that double flicks require a grip shift compared to single flicks due to my home grip (which I am continually working on), and in the cases where I'm much slower at double flicks (e.g., on bigger cubes), it's more a technical issue rather than anything else to do with physical coordination/ability potential. Most OH specialists won't be double flicking every U2' that presents itself, even though it's possible to double flick a U2' faster in isolation even when doing OH. So in reality, double flicking isn't *always* faster in the context of a full solve. Of course, this doesn't mean we shouldn't learn them - but this is a nuance I feel that many are missing.
As soon as I start trying to double flick more during solves, I find my level of creativity improves even though tps isn't really improved by all that much. It's more a flow thing that helps with lookahead, and I suspect that's really the main benefit of double flicks for most cubers who need to improve at looking ahead.
To the original question of how Max manages to be so fast with single flicks - well, double flicks don't make up most of a solve, and if my hypothesis is correct that double flicks help to improve lookahead more than tps, then it'd make sense that someone with already world-class lookahead might not have all that much to gain from switching to double flicks. I think double flicks help to set up the next moves and prepare your mind to move onto the next triggers whereas single flicks cause a small interruption in the brain because it ends up being almost like "flick think/pause flick". If one were to record a solve and frame count one with double flicks vs single flicks, I don't imagine there'd be such a difference between U2' vs U' U', but it *will* affect how fluid an overall solve becomes.
Unconvinced? Think about eido U2s - objectively the fastest type of U2, yet why don't we always use them? The answer is because they can mess up the grip for the next moves, or that it's not always easy to get into the right grip to initiate an eido U2. eido U2 also requires an exact starting grip to initiate properly - it's not so much about dexterity or physical ability, but rather, technique and correct preparation in setting up for it - not so dissimilar to double flicks really.