Yeah, easy. But AFAIK, unfortunately Wine could not just reimplement the Windows Kernel interface, since Windows DLLs are subject to copyright (so you need permission to redistribute them, guess by whom?), and lots of them are needed for a "working" system. That's why they set out to also create a full set of compatible DLLs for each subsystem instead - a monumental task involving reverse-engineering [often poorly documented] stuff, and reimplementing the thing [possibly without infringing any patents]...

BTW Wine is approaching release 2.0. Hooray for Wine!