There are hardware requirements to run hardware-based tessellation, direct compute 11/multi core, as well as many new things with DX 11 API. The previous card do not have the technical capabilities (hardware) to carry on these types of processes. No amount of drivers can force hardware into existence.
Second forcing GPU upgrades is NOT in Microsoft's best interests. As it was shown with Dx9-Dx10. Many people now wont even upgrade GPU's to Dx11 Hardware and gamers are a limited market when it comes to PC's.