You're not exactly wrong – I don't serve out a json feed myself, even – but RSS/Atom are terribly underused for cases other than "personal organization of feed reading" when they could handle a lot of weirder poll-for-updates automation, and the json format seems like an attempt to encourage that direction. You don't need a bespoke API if what you have makes sense in a feed like this, but people would really rather work with a json than XML.

For me personally, two reasons

1. I find it easier to work with JSON using javascript

2. The stupid reason, I find it more aesthetically pleasing

XML is a far more complex format to parse.

JSON is more readable to humans, while still being great for machines