Same reason as SOAP/WSDL: there's good money in consulting so it would be stupid to do something that to simple, people might be able to solve their own problems.

Honestly I get that there an advantage in respect to tooling and this might ease integration into existing system, but I can't help feel that this is introducing an extra level of complexity that you would only find acceptable if you at IBM customer level scale.

The ability to introduce at least some type safety seems nice though.