It seems rude to me and it doesn't help me avoid getting my own questions closed for obscure unwritten rules
What obscure, unwritten rules? The rules for asking questions are all outlined in the FAQ.
So if none of the people who closed the question comment on it to state why
They generally don't need to comment on the reason why, because when they vote to close the question, they select the reason for closing. You can view all the descriptions if you press the flag button. Here they are:
Very low quality: This question has severe formatting or content problems. This question is unlikely to be salvageable through editing, and might need to be removed.
Exact duplicate: This question covers exactly the same content as earlier questions on this topic; its answers may be merged with another identical question.
Off topic: Questions on Meta Stack Overflow are expected to generally relate to the Stack Exchange family of websites and/or community in some way, within the scope defined in the faq.
Not constructive: As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or specific expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, see the FAQ for guidance.
Too localized: This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.
Spam: This question is effectively an advertisement with no disclosure. It is not useful or relevant, but promotional.
Not welcome in community: This question contains content that a reasonable person would consider offensive, abusive, or hate speech.
However, there are some instances where I can imagine these descriptions leaving something to be desired, especially on borderline questions. In these situations, it would be nice for the closers to comment on the reason why they're closing the question, although I don't think that action should be expected. If a pronunciation question was closed, the asker could probably infer that it was because pronunciation questions are not allowed on Stack Overflow, even without comments explaining so. Furthermore, if they read the description for "not constructive," they could probably determine that the question was closed because it could lead to "debate, arguments, polling, or extended discussion."