I believe this is due to ADA Title III requirements for publicly accessible businesses:

The ADA requires that businesses open to the public provide full and equal enjoyment of their goods, services, facilities, privileges, advantages, or accommodations to people with disabilities. Businesses open to the public must take steps to provide appropriate communication aids and services (often called “auxiliary aids and services”) where necessary to make sure they effectively communicate with individuals with disabilities.

www.ada.gov/resources/web-guidance/

Screen readers and braille displays can’t do anything with all the fancy JavaScript and other nonsense that frames and formats the content for delivery, including the user-identifying-and-blocking stuff. The website is legally required to make the content accessible in a way that assistive technologies can use, and usually the simplest way to do that is to just deliver plain text. I think “reader mode” is leveraging this functionality to get the content in a simple form without the cruft, basically just telling the website that the user needs the ADA compliant version.