I think "beloved" is a bit of a stretch... tolerated perhaps, for their simplicity and versatility, in that you can make them work for most kinds of data transfer and configuration if you don't mind shoehorning your needs into that spare box. But beloved? hardly...
Having written quite a bit of open API specs, I don't agree with you. Json is hard to read, yaml has own quirks, especially when you try to spilt it into parts.
Amazon also tries to invent own language for describing apis, so I guess they are not happy with open API too.
Anyway, without ability to generate code from spec, there is not much use from it. Code gen/nswagger/open API generator and others produce terrible code, at least for java/c#/typescript(there's 4.1k open issues for open API generator), using custom generators for codegen make problem less painful, but that is additional burden, I'm looking for better alternative, would be very interesting to see what they will do with code generation.
Codegen is coming online as we speak. We do codegen from TypeSpec in Azure across multiple languages, and the results are pretty great. We're moving that over to the TypeSpec project so everyone can generate code. Obviously my opinion is biased, but I think the results are significantly better than what you find elsewhere in the ecosystem.
I would be absolutely stunned if anyone, ever, has described OpenAPI as "beloved". It does its job, but that's about the best you can say about it.
Yaml is beloved when it's maybe two pages of little indented config.
It becomes a nightmare once people try to emulate a DSL with it.
Just because you don’t have the problem it’s solving doesn’t mean the problem doesn’t exist.