We close questions because they cannot or should not be answered:
- Duplicate: we don't want to answer the question because it's already been asked and answers before.
- Off-topic: we don't want to answer because the question is outside our area of expertise, so answers cannot be trusted.
- Unclear: we can't answer because the it isn't clear what the question is about.
- Too broad: it would be unreasonable to expect a decent answer because there's too much to say.
- Primarily opinion-based: answers would be a soapbox for the answerers rather than helpful advice.
None of this has to do with effort. It's possible to have questions which are on-topic and would lead to reasonably-scoped, interesting answers, with the asker spending zero effort. Copying a question from elsewhere is an obvious example. It makes no sense to arbitrarily prevent questions from being answered based on an unavoidably subjective assessment of the state of the asker's mind. Closure is based solely on the question's content, and specifically on whether it is answerable.
There is a tool on Stack Exchange whose description is
This question does not show any research effort; it is unclear or not useful
This is downvoting. “Shows too little effort” is a valid reason to downvote.
There is some correlation between a lack of effort and an unanswerable question. For example, if a question is unclear, then both closure and downvoting are suggested. But there is a reason why closure and downvoting are different tools. Some questions warrant one and not the other.
We only close homework questions when they are not answerable. You are not supposed to close a homework question because the asker was lazy. Common reasons to close homework questions include:
- The question doesn't stand on its own. For example, it's a continuation of a previous exercise, or of material seen in the lecture, and can't be understood without it.
- The question lacks information about what answers can assume. While it would be possible to answer it, the answer may end up going well over the asker's head, using theorems that the asker is supposed to derive from first principles, etc.
- The question is an exercise that requires applying several concepts in a straightforward way. Explaining one of the concepts involved would be ok, but explaining all of them would be too much.
- The question is a scan of text. This is an exception — we have a consensus that readers aren't supposed to decipher the text on images.
If you think a question is valid but the asker was lazy, the correct behavior is to downvote the question but leave it open.
There was a similar debate about Stack Overflow a while ago. You can read my thoughts here and here, and Shog9's conclusion.