I posed my first Math Stack question, earlier this evening. It was on the product of expectation values. I received an answer that enabled me to solve my problem by using Cauchy's Inequality. The respondent referred to Jensen's Inequality, a more comprehensive relationship than Cauchy's as I found from reading a Wikipedia article. To understand the Wiki article, I should have far more mathematical background than I do: measure theory, for example. Why not urge respondents to gauge the level of their replies to the level of the questioner? I am sure that my statement makes clear that I am not an advanced graduate student in mathematics. If I were, I would not have needed to pose the question in the first place.
In short, the answer is "because we don't exist to help only you." The purpose of StackExchange is not to answer every question someone has as they encounter them, but to produce a high-quality database of questions and answers.
You may want to read this meta question. We don't answer questions to help you; rather, we answer questions to help anyone who may have a similar question. As such, it's perfectly fine to answer with higher level mathematics than the person asking. Sure, most answerers will try to keep on the questioner's level, but there is no requirement to do so.