Why should redux action types be in UPPER_CASE?

capital letters for constants

It is just a convention which is common also in other other languages.

Interesting article on redux naming conventions: https://decembersoft.com/posts/a-simple-naming-convention-for-action-creators-in-redux-js/

It is mainly because of actions is being refereed from multiple modules, be it saga or react ui. It is just the convention, and gives easy readability in code. In ReactJs, actions are important focus as they are meant to change the state of the reactUI.