I have actually written code to produce LLVM IR to conform to a platform's native C ABI. It's quite a bit more work than "just a bit of register/stack allocation". For one thing, LLVM IR doesn't expose the concept of a stack pointer, so you can't just put things on the stack where they need to go. You need to put things in LLVM's abstractions in places where you happen to know the codegen will lower them to where they need to go. And it's different for every platform.
In the relative context of implementing a production runtime, that's "just a bit of register/stack allocation".
If you're hobby-hacking a project, then you may as well just target something that already exists, because micro-inefficiencies that matter on the scale of a deployed production platform probably don't matter for your human-resource-constrained hobby project.
If you're designing the next browser language runtime, or a language meant to be used outside of hobby deployments, there's no point in permanently hamstringing your implementation architecture to save a small amount of implementation time within what is already a large and complex problem space.
