It is also a format that can be read as-is without any preprocessing. In some cases base64 can do that, and this format does make heavy use of base64 varints.

Sure, you can encode as JSON, then compress with gzip and then base64 encode. You'll probably end up with something smaller than rx and be extremely safe to copy-paste. But your consumers are going to consume orders of magnitude more CPU reading data from this document.

RX is usable as-is, is compressed, and is copy-pasteable. It's the unique combination of properties that makes it interesting.