Why is user-defined Javascript avoided by Google's AMP?

Javascript is not prohibited by AMP ( from Ampproject.com or Ampproject.org ), but it severely restricts the use of JavaScript. However, user-defined JavaScript is a strict no. The main AMP framework is designed with JavaScript.

As approved by the project, AMP permits custom white-listed JavaScripts. Read more on  How Google AMP improves the SEO of your website?

JS, jQuery and other JS tools are not allowed. You can still link them to the website, but the website will not be AMP compatible.

The AMP project is continually developing and is now introducing a range of features.

The core idea of AMPs is speed, which causes one to think differently about how we design websites.