I think you missed a vital reason and that is portability and sharability. No one needs to install anything special to read or edit a text file.
It's the digital equivalent of paper and pen when you think about it and that makes it a popular platform for everyone to be able to consume it.
Plus, back when we were all old and dinosaurs roamed the Earth, the text file was the lightest/smallest way to transfer data (read: ideas) between computers.
The practice/behaviour saturated the industry so much that many readmes of today are still on text files.
Maybe there's a correlation to the real world, where we're definitely more apt to consume mediums that don't have superfluous data points than the words/ideas that they're meant to convey? Maybe I'm just talking out of my ass?
Either way, plain-text resources are the easiest to create, share, and consume because there's almost a universality in the standard for text files (except the EOL and CRLF but that's more of an inconvenience than anything).