Because the M3.. is a BMW...optimized. Kind of like a Taurus SHO.. Taurus came first.. then added goodies and badging. Its a totally different concept.
Why doesn't a Corvette look like a Malibu? Camaro look like a Malibu?.. etc.
My point is that the whole signature look across models has been done for years. Pontiac, Audi, Mercedes, etc. Most of the GM brands had their own looks. That's how they could sell the exact same car as different models.