One reason is that shaders are stored as text and require each driver vendor to write parsers for them and not everyone implements them in the same way.
Further, the drivers have to be more permissive than necessary. It's not an option for a GPU vendor to ship a driver update that would break Doom/Quake/Dota/etc.
Ideally games would ship only shaders that are strictly valid GLSL but that's not quite the case.
If you're developing GL, do everyone a favor and start using the official GLSL validator in your build scripts.
