Both of those premises often do seem to be true. Most answerers, especially with limited experience with high-quality Q&A, are unable, or at times unwilling, to figure out if a question has show-stopper problems. Some of these problems can be fairly subtle. While many answerers probably would want to make some effort to match answers to changed questions, that opens up further issues: how, exactly, are they to do so, and what are they to do about chameleon questions? You can’t expect someone to babysit their answers by manually checking on them for the next few days, so that means you’re looking at notifying them if the question is edited (or if the edits reach a certain threshold of significance). That’s inefficient and obnoxious, and the natural response to getting an irritating stream of notifications that require more work out of you is to try to cut them off as quickly as possible. So answerers have an incentive to stop caring about the question, and stop getting pinged about it, as soon as they can expect that further votes on their answer are unlikely. Also, many answerers are drive-by users, that may not be interested in any further site interactions, and leaving low-quality questions open to attract borderline spam, chattiness, and other moderation drains is not helpful.
Finally, as I see it, the basic point of closure is to forcibly align the incentives of the asker with the incentives of the site. They want answers with as little effort as possible, especially including at least one that they see as good, but extras don’t hurt much; the site wants verifiably high-quality answers only, and whatever effort is needed to make the question good enough to attract those. Closure ensures that the asker wants to satisfy the site’s expectations about question quality (and therefore answer quality and conciseness) in order to get anything. (This does not, of course, mean that they need to single-handedly fix the question. It just means they are no longer trying to skirt quality requirements they don’t understand and don’t care about.)
This still leaves the problem that askers may see bargaining with close-voters as an easier way to get answers than actually fixing the question, or may try re-asking it. The second can be managed with rate limits, dupe detection (AI flagging?), and so forth; the first is partly handled by emphasizing next steps in an objective, detailed, but straightforward close message, while de-emphasizing individual voters.