There are reasons for that though. The two OSes may look similar on the outside, but they are vastly different on the inside. The new core that enables so many cool features (multiple CPU cores, better graphics) is shared with Windows 8 RT (based on Windows NT) and not Windows Phone 7 (based on Windows CE, same as Windows Mobile).
What they are getting from the WP8 is the Windows Phone 7.8 update, which incorporates the new start screen - it's the most noticeable change in the OS and will actually benefit the current Windows Phone smartphones.
Also, users who move to the new OS will get to keep using their Windows Phone 7 apps as those are designed to work on WP8. There's no backwards compatibility for the newly developed WP8 apps though, and there's nothing that can be done about it - for example, the Adreno 205 just won't cut it for games intended for a Snapdragon S4 (which packs an Adreno 225 or 305). Or else your Lumia would have to grow itself or something. But it cannot, because only Apple can do that.