His main reason is that it is an operational language (like C or Python) and not a declarative one (like HTML or Markdown or Wiki Markup).
It is possible to write a document using only the markup-like subset of LaTeX, never defining any new commands or environments, and avoiding any commands or environments that assume that the output is to be a printed document. If you absolutely have to use LaTeX to prepare an academic paper, this is the best thing to do, and in academic disciplines where LaTeX is routinely used, graduate students are taught to do this when they learn to use LaTeX. The trouble is that this removes all of the flexibility and power that makes LaTeX so fun to work with. And for most purposes (i.e. unless you make extensive use of LaTeX math mode) it is easier to write a document in a declarative markup language such as Markdown which can readily be converted into LaTeX (but not of course vice versa).
Talking especially to philosophers, he finds: "The use of LaTeX to prepare ones academic papers is just another instance of this phenomenon - giving ones papers the outward appearance of a paper in physics or mathematics in order to bask in the glow of intellectual seriousness and indispensability that we attach to those disciplines. But using LaTeX does not make you a mathematician any more than building a runway makes you an airport."