It’s the default because Vercel dumped millions of dollars into devrel and marketing. It’s not and never was definitely better than other available options.
Good docs? Have you tried to get definitive answers on image optimisation, response modification, error codes, or much anything other than “serve these react components at this path”?
When there is documentation, it’s often a mishmash of incomplete examples and links between the two totally incompatible editions of the framework.