The important point of the phrase is "within the scope defined in the help center." It means that the question is (perceived as) not suitable for the site. What is suitable is explained at https://math.stackexchange.com/help/on-topic, which is also linked in the sentence you mentioned.
Moreover, usually, a more specific reason is provided, such as:
This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
However, this more detailed description is not directly shown on some mobile devices.
Thus, "off-topic" does not mean "not about mathematics" but instead "not suitable for this site."
I hope this was the type of answer you were looking for. If instead you want to know why many questions are seen as not suitable for this site, then this is a broad subject and you best would narrow down the scope by providing some examples of questions.