>what's really funny here is how absolutely horrified people are by the suggestion a single company which has a monopoly shouldn't also define the web platform

They don't. In general browser specs are defined via various standards groups like WHATWG. As far as I know there is no standard for what image formats must be supported on a web browser,[0] which is why in this one case any browser can decide to support an image format or not.

[0] https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...