I was on here just now posting another question and it struck me how many questions there are on here about 'why was this question closed'. It got me to thinking: should we make it mandatory for people who vote to close a question provide the reason for it. Now I know this already happens in some instances where people add extra comments discussing it, or in a dupe where the duplicate is linked, but it is evident that not everyone understands why their question has been closed and they start clogging up this site looking for answers. Hence should we make it compulsory to make VTCers explain the specific reason why they think this question needs to be closed/put on hold if it's not a dupe?
There are a few problems with requiring people who vote to close to post a reason:
People won't do it, and bad questions will stay open.
On other SE sites where there's lots of voting to close, you rarely see the custom off-topic close reason used because it takes more time than it's worth to type out a more specific explanation.
It's a waste of people's time if one of the existing close reasons is a good explanation.
The standard close reasons are a bit generic, but they've been developed over 5 years and millions of questions and they actually fit most questions that get closed quite well. So requiring people to come up with something better on the fly is asking too much.
It takes 5 votes to close, so you'd end up with 5 slightly (or wildly) different explanations of what's wrong with the question.
And if you allow any sort of "choose from boilerplate" then the same objection you raise is going to come up: someone will say the boilerplate doesn't apply to their particular question.
Finally "clogging up this site" (meta) isn't an issue - that's one of meta's purposes: to give people a place to ask for more information about how the site operates. We've seen it just today with an extended discussion about whether a certain question should or shouldn't be closed.