But that presumes that there is always that one answer that helped. But in my experience, often (mostly, but not exclusively, in writing and worldbuilding) it is very hard to select one answer, because several answers give several pieces to the ultimate solution.
I’m now too lazy to search for one of those occurrences, therefore let me give a made-up programming example: Say you asked ”How do I achieve foo in language X”. Now you get one answer that gives a functional solution that almost fits your needs, but not enough explanation that you could fix it. Another answer gives an in-depth explanation, but no concrete solution to your problem. However thanks to that explanation you are able to adapt the solution of the first answer to your needs.
So which answer was it that helped you? The first one, giving the almost-solution? Well, it certainly helped (because it was what you derived your ultimate solution from), but had you only been given that answer, you wouldn’t have solved your problem.
Or was it the second one, with the in-depth explanation? Well, sure, that helped too, but then, without having the almost-sufficient solution from the first, you would have spent much more time getting to a solution based on that information alone.
Thus it was both posts together that helped you get the solution to your problem. But the accept mechanism demands that you choose only one.
I do agree that it is useful to mark a question as resolved. I’m not sure it is useful to mark an answer as the one resolving it.