[Someone]

Partly it’s commoditization (https://en.m.wikipedia.org/wiki/Commoditization). There’s no money in differentiation of image formats (in part because browsers dictate what formats people can view, but also because the current set is good enough for typical use cases). Note taking apps seem to think there is in note taking.

Partly it’s not true. Tools such as Photoshop have their own image format. You could even claim PDF is the format we converged on for exchanging rich text data.