an emulator fools a program into thinking that it's running on a particular system or native hardware, the emulator to the program is the entire machine which is essentially what it's trying to be. if we look in the dictionary for the meaning of what an emulator is you'll find this
Computer Science. To imitate the function of (another system), as by modifications to hardware or software that allow the imitating system to accept the same data, execute the same programs, and achieve the same results as the imitated system.
that certainly sounds like what wine is doing, but it's not, pretty much any software that doesn't use any API calls can be run on linux, or with very little porting involved, what wine is trying to be is not an emulator, but the API compatibility layer so that the calls for a API function can be translated properly, just as the API library in windows currently does.

the programs themselves run on the processor and run just like any program under windows would, that's what allows them to run in linux and not suffer any speed loss as a result, any loss suffered is due to the fact wine is far from perfect, but they've done a real nice job so far.
a driver takes a call from the operating system and then translates in into something the hardware can understand, and visa versa, this doesn't mean the driver is emulating anything, in the most simplist term a driver is a compatibility layer between the hardware and software, this is what wine is, a software compatibility layer.

hopefully that will shed some light on the fact as to why wine is not an emulator