Because those frameworks are easy at first glance. Adding some interactions with React is easier than with jQuery. Even better if you could make the whole page a React app, then you can add those 100s libraries to do…stuff. Optimizing a React app is hard and it will probably require some deep thinking about global state and its modification and we don't have time for that /s.

By then, the app is built and running. Even though the code is a mess because the developer only know about React, nothing about the DOM and software architecture.