Isn't it a bit rude to give someone a vague error message rather than explain the problem in some detail?
Well, yes. Normally, the designers of software try to go out of their way to help users get around syntax errors. It actually is an important teaching moment.
But the odds that the problem with a question on Stack Exchange consists merely in the syntax are vanishingly small. In computer code, the difference between a functional program and a bug can be as small as a single semicolon. But in human language, the difference between a clear, interesting question and a bad question tends to be lots of individual edits and maybe a total rewrite. Most low-quality questions on Stack Exchange contain content or semantic errors.
The developers of the Stack Exchange engine haven't developed an algorithm to detect content problems (probably because that's impossible), but they believe that certain syntactical indicators correlate with low quality questions. They want to encourage you to look over everything about your question, not just the syntax, and find ways to improve its quality.
As a community, we understand your frustration. We wish it weren't necessary to filter out questions at all. However, since there are thousands of new questions added across the network each day, the developers put speed bumps in front of askers so that our top users can keep up with the flood. It's really in your best interest to follow the advice found in the accepted answer if you want to stand out (positively) from the crowd and have your question answered.