So, legally speaking, no website or app is _required_ by the Americans with Disabilites Act (ADA) to be made accessible. Title III of the ADA is the part that deals with public businesses and it doesn't mention digital services at all, but has been interpreted by various courts to include websites, there was a notable case against Dominos about this, wherein the judge ruled that Dominos did, in fact, have to make their online ordering tool fully accessible to people using screen readers, but this was the judge's understanding of the law and hasn't been challenged at a higher court. There's also section 508 of Title II of the ADA that does require government websites to be accessible, but it imposes no such requirements on private businesses. The DOJ apparently was considering a formal guidance on this a few years ago, but it never materialized.
Here's a quote from an article I'll link below, this article seems up to date with my knowledge of the matter and I think will help answer most of your questions. (Even if that answer is "lol, who knows"):
> ADA legislation as is applies to websites is currently a gray area. This often leaves the interpretation of the law up to the court where the lawsuit is filed, generally a state court. Suits have been filed in every state, though the majority of cases are in New York, Florida, California, Pennsylvania and Massachusetts.
>
> Do all websites have to be ADA compliant? That depends. Courts generally reach one of the following conclusions:
>
> Yes, a business website is considered a “public accommodation” and must be accessible.
> Yes, but only if the business also has a physical location that serves the public.
> No, the ADA does not specifically address websites and therefore does not apply.
https://www.accessiblemetrics.com/blog/do-all-websites-have-...
So, the answer is that there are several good reasons to make your site and/or app accessible: You should do it because it's the right thing to do, it improves your ranking in google (a site that can be screen read can be mechanically crawled easier and is viewed favorably by Google's search algorithm), and there may come a day in the near future where it is mandated.
Honestly, I'd recommend reading through the WCAG standards sometime. It's a very well thought out document that lays out pretty straight forward approaches to making your digital content more accessible to users with a wide variety of disabilities. When people think 'accessibility' they often are only thinking about blind or vision impaired users, but there's so much more to it than that, and a lot of what you do as you build an app or site affects accessibility, whether you know it or not.
Dyslexic people have issues with some fonts, for instance, I don't think it's unreasonable to build your site in a way that lets the user override the font using their browser's settings. Or for people with limited motor control, placing frequently used together controls nearer to each other can improve both your UX and your accessibility for people with motor impairments.
I realize this isn't the easy, straightforward answer you wanted, but this is the reality of the situation.