If you read through the case history, you can see that the ADA does not have specific language saying something like "websites above size x must be accessible, and that means a, b, and c need to be followed." Much of that has been developed into case law and precedent by judges ruling in numerous lawsuits over the years, which is why it's still a somewhat grey area for edge cases.

And that's why a ton of video lectures from UC Berkeley had to be taken down due to lack of captions.

https://news.ycombinator.com/item?id=13768856

The spirit of the law is good, but sometimes it just means that we lose "good enough" in pursuit of "perfect".