Protobuf's main design goal is to make space-optimized binary tag-length-value encoding easy. The mentality is kinda like "who cares what the API looks like as long as it can support anything you want to do with TLV encoding and has great performance." Things like oneofs and maps are best understood as slightly different ways of creating TLV fields in a message, rather than pieces of a comprehensive modern type system. The provided types are simply the necessary and sufficient elements to model any fuller type system using TLV.

Yes but the point is that nobody outside of super big tech has a need to optimize a few bytes here and there at the expense of atrocious devx.