Maybe I'm dumb or missing something, but why not just do what Google does -- convert all inbound HTTP requests into Protobufs, separate out the internal stuffs

Maybe I'm dumb or missing something, but why not just do what Google does -- convert all inbound HTTP requests into Protobufs, separate out the internal stuffs from the external stuffs, run it through your myriad of internal services, and then on the way out you still have your nicely delineated protobufs which you can convert back into HTTP. Why are we mucking about with headers in the first place?