These sorts of things need to be as simple as humanly possibly and get out of your way so you can actually do some programming. Instead protobufs are the opposite. Full of backwards compat issues, footguns, and needless complexity that make them an absolute horror to work with.
It's like comparing apples and screwdrivers.
Library/module/API developers often seek to deliver maximimum adaptibility in their interfaces so that consumers of them can satisfy their own more specific/pressing requirements.
We have a bunch of apis like this where I work. It's a huge pain, tbh. You lose the ability to just curl endpoints, or to see what data came over the wire for debugging, not to mention having to use esoteric client libs because every off-the-shelf one just assumes json over http.
With protobuf the schema is essential, it is the foundation. I feel this is a huge advantage especially for communication protocols.
So you have 2 teams do a meeting, we work out a contract we like, dump that into a jsonschema and you can enforce by unit tests that schema is respected on each side (producer/consumer)