(Note that the same principle is followed by answers. The accepted answer may have zero votes, or fewer votes than other answers.)
I think the idea is that the community over time can up-vote useful questions and answers, independent of the intent of the original question and the accepted answer.
All questions are presumably useful to at least one person. Upvoted questions form a dynamic FAQ. But over time, some questions may become less useful, or irrelevant, as Unity and tools that serve it evolve. The voting maintains the FAQ.
Or maybe the answer is simply: since most questions get answered, giving them all one vote for getting answered serves no purpose.
I notice that you ask that the question - not the answer - gets at least one vote if there's an accepted answer.
I don't think that follows, because even though the question got answered, it could still be an uninteresting question. For example, it could be rather off-topic, or it could be so specific to a specific project that it isn't likely to be of help to anyone else.
Although I generally upvote any answer I accept, it seems possible that an answer could be the one that resolves your question, even though it's not a particularly good answer.