Yes, the directx level is based on hardware, as well as software. The video card must be hardware capable of dx11 to use dx11. GTX6xx series are hardware capable of dx11.1, but earlier cards can not use dx11.1. Cards earlier than GTX4xx series, can not use dx11, only dx10. And so on.
Edit: if you are asking why the gpu does the work instead of the cpu, it's because modern graphics require a lot of power. A gpu is a 256bit cpu with ram running at 4000-5000mhz. It's basically a self contained super-computer, but it's main purpose is graphics rendering. It offloads a huge amount of work from the cpu.