They have no bearing on each other, so there's no reason one should affect the other. Just like we can vote regardless of close status, and close regardless of votes, accepted answers and close status are also just totally separate.
Being closed doesn't prevent any existing answer from actually winding up solving the answer's question. Having information that it solved their problem is useful for future visitors and we wouldn't want to prevent ourselves from getting that.
if the question isn't stack ready yet
Closure doesn't indicate it's not stack ready yet, it indicates five or so people thought it was. They may be completely wrong, and the question can get reopened.
Also, questions might be stack-ready never, because they're looking for opinions, or off-topic, etc. Still no harm in getting an accepted answer marked in that situation.
You mention in comments:
but the main difference is closure vs Hold
“On Hold” is the same as Closed. It was just a new name introduced to convey the temporary and non-punitive nature of our question closures to newbies, who are used to forums where questions being closed is permanent and a punishment. (Well, also, an edit to an on-hold question can bump it into the reopen review queue where an edit to a closed question won't do the same, but that's it.)