Why doesn't this site have MathJax activated? I've seen several questions and a few answers that are trying to use it, but it obviously isn't.
Because sites do not have MathJax enabled unless the demand is great enough (as it adds load.) We are nowhere near the usual threshold for SE to enable this.
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1To add a bit more detail, the problem with MathJax and the current way SE implements it, it is loaded on every single page load for a site, whether it is used or not. This means that there is a human detectable delay in page load times for every page on sites where it is active. – AJ Henderson♦ Jul 11 '18 at 14:19