Launching kernels manually is an error prone PITA which I believe is the principle reason for CUDA's popularity. Having the compiler give an error when you mess up is a huge benefit. But having the compiler allow you to express "I want to launch this kernel over a grid with these dimensions, with these arguments" as a single expression is where the vast majority of the value comes from.

The having it all in a single file is mostly an artefact of the fact that it is C++, because C++ is single file at a time compilation. In D (which is multiple files in a single compiler invocation) with DCompute (which targets CUDA and OpenCL with upcoming support for Vulkan and Metal), you are required to write the kernels in a separate module, but you get all the benefits of the compiler complaining when you mess up _and_ the expressivity of "launch me this kernel".