We allow only good questions, but not all good questions

On the Stack Exchange Q&A network, every site defines its scope as an "on-topic" page, sometimes augmented with community-written and -maintained documents ("meta posts"). There's also a snappy tagline for each site that describes it to Internet passersby, for example:

Super User is a question and answer site for computer enthusiasts and power users.

With notable frequency, I see people (who clearly didn't read the on-topic page, despite it being linked from the Ask Question page and the tour) ask about a topic that sounds like it would fit in the short description, but is not allowed by the actual on-topic guidelines. Such questions are closed or migrated to an appropriate site.

After an experienced user explains to the poster via comment why the question is off-topic (or when the question gets closed), the poster tends to defend the post, saying that it's about insert short topic summary here, clearly not having read the articles that usually get linked by the experienced users. Then follows a back-and-forth of about four rounds in which the experienced user explains that, despite the question possibly sounding on-topic from the summary, the community does not accept it.

The thing that the new users don't understand is that the community defined its topic and that said topic can't be fully delimited by a short tagline. Certain types of questions are disallowed (e.g. software recommendations and web app questions on Super User) because - even though useful information might come of them - they tend to attract problems, or are otherwise not well served by the Q&A format. To keep the quality of all posts high, it's necessary to reject some high-quality posts that are outside the defined topic.