Because the law doesn't stipulate which methods you use to make something accessible to people with disabilities. It just requires that everyone have equal access. To litigate, someone has to show that they couldn't access something, and that the failure to access it caused them some measurable harm. In a couple years you can show a judge that a free LLM screen reader could have solved their access issues, and my guess is that those cases will then be thrown out, and the predatory law firms will move on to something juicier.