Most importantly, the whole point of having a driver do compilation for you is compatibility. AMD's GCN, for example, currently has three different generations out, and there are small differences in the set of supported instructions and in the instruction encoding.

Not having to deal with legacy stuff in instruction decode is one of the big advantages GPUs have over CPUs, and it relies on having the driver do the compilation.

All that said, it's nice that people are having fun with AMD's and Intel's GPU ISA, but honestly, why on Earth do they do it on Windows? There are perfectly fine open source drivers available on Linux that you can easily modify to play around with the ISAs as much as you wish.