You don't need a text editor, you just need to be able to read a sequence of bytes. XML anchors at the almost lowest possible point: you have the stream of characters. It's a pointless exercise to consider a format except on the premise that you have access to data in some way, and sequence of bytes is the simplest possible abstraction of data, a sequence of characters the second simplest.

ASCII encoded text is simpler than UTF-8, but there is content that can't be represented in ASCII. Zip comes a bit up the food chain from there, and SQLite comes quite a bit further.

XML is about being meaningful without tooling at the lowest possible level. Any added required tooling or documentation by definition makes the data less 'open'.