Because header files are common and used across multiple projects to create a common environment or API between those projects. The headers are a shared space, the code that calls the headers (e.g.: the platform-specific RISC V implementation) is a less shared space and may be outside of Linus’ domain, so to speak. Basically, it’s like if two cities shared a reservoir for drinking water. If one city decides to mix raw sewage in the water at their pump station, that’s a bad idea, but it’s a problem for that city to deal with. If, on the other hand, it dumps the raw sewage into the shared lake, it affects the other city as well