Why is it the responsibility of the programmer to call UI related methods on the main thread with:
DispatchQueue.main.async {}
Theoretically, couldn’t this be left up to the compiler or some other agent to determine?
The actual answer is developer inertia and grandfathering.
The Cocoa UI API is huge—nay, gigantic. It has also been in continuous development since the 1990's.
Back when I was a youth and there were no multi-core, 64-bit, anything, 99.999% of all applications ran on the main thread. Period. (The original Mac OS, pre-OS X, didn't even have threads.)
Later, a few specialized tasks could be run on background threads, but largely apps still ran on the main thread.
Fast forward to today where it's trivial to dispatch thousands of tasks for background execution and CPUs can run 30 or more current threads, it's easy to say "hey, why doesn't the compiler/API/OS handle this main-thread thing for me?" But what's nigh on impossible is re-engineering four decades of Cocoa code and apps to make that work.
There are—I'm going to say—hundreds of millions of lines of code that all assume UI calls are executing concurrently on the main thread. As others have pointed out, there is no cleaver switch or pre-processor that's going to undo all of those assumptions, fix all of those potential deadlocks, etc.
(Heck, if the compiler could figure this kind of stuff out we wouldn't even have to write multi-threaded code; you'd just let the compiler slice up your code so it runs concurrently.)
Finally, such a change just isn't worth the effort. I do Cocoa development full time and the number of times I have to deal with the "update control from a background thread problem" occurs, at most, once a week or so. There's no development cost-benefit analysis that's going to dedicate a million man-hours to solving a problem that already has a straight forward solution.
Now if you were developing a new, modern, UI API from scratch, you'd simply make the entire UI framework thread safe and whole question goes away. And maybe Apple has a brand new, redesigned-from-the-ground-up, UI framework in a lab somewhere that does that. But that's the only way I see something like this happening.