Cynically-but-accurately: it's because this problem is in the absolute infancy in terms of being understood, the science is almost nonexistent, and it's taken what, 20 years for sites to understand what alt text is and why they should encourage people to specify it and that's a lot easier to explain, something this nebulous is gonna be a lot harder.

You can make things like "your color contrast must be within this window of numbers" and "don't include anything that flashes more than 3 times per second" as a hard-and-fast rule that accessibility checkers can check for, and therefore that sites can apply without having to understand the neurology behind, but 90% of this problem is guesswork and entrail-reading based on cutting edge neurological research that you really have to look to find. I will not say I'm an expert, but I know more than most people who aren't studying this sort of thing directly, and we still hit The Wrong Window; most sites that don't trigger problems for people are not triggering problems for people completely accidentally.