From my experience, it's because some of the people that would need to be doing that have their heads in the sand about e.g. the necessity of middleboxes and application-layer proxies. Network-layer folks still seem to think that the Internet must not have application-layer proxies, and some actively resist good efforts to expand the application-layer protocols to better deal with the reality of application-layer middleboxes, such as those widely used in IoT applications. And some try to eliminate good mechanisms that otherwise would work, such as CONNECT over TLS.