There are a variety of reasons:

* Safety - not everyone protects their website from XSS attacks

* Speed - Downloading 30 or more script files from all over the planet slows down browsing

* Tracking/privacy - it doesn't make your browsing private, it cuts down the more obvious things - https://www.youtube.com/watch?v=f_f5wNw-2c0

* Principle - browsers render HTML - nobody should be forced to require javascript ONLY to read the content or just to have access to the correct page. If you have fancy effects or some additional functionality or if it's required to provide core functionality (e.g. maps) then fine, but JUST rendering text?

Short version: principle of least privilege.

I crank down perms and open them as little as possible, for as brief a time as possible.

If your site's annoying enough to visit, I won't.

Too many slow javascripts on too many pages, the web is ridiculous these days.

Costs >> Benefits