In computer programm single threaded programs are pretty predictable (apart from human errors). As soon as you have multi threading that goes out the window. Modern CPUs in most devices you use have what's called a scheduler that schedules when to let different things actually use the CPU so you can actually do multiple things at once. It's a super important concept for what we want to do with devices. But because of that you have no guarantee about when (or if) other threads of your own code will execute. Apart from truly insane edge cases, single threaded programs act pretty deterministically. Multi threaded ones do not. It's very similar to the "it's alive and dead until you check" idea because you just don't know. So much so that there are data types we use called things like Maybe where the result is either a success or a failure and you write code for both.

Also much like the cat in a box thing, programmers don't really view it as magic, it's just sort of a side effect of the uncertainty.