How can Wilson Audio speakers sound that good if they are using OEM drivers?

OEM is the wrong term. Original equipment manufacturer applies to parts that are manufactured by the producer of the finished product. Your question then means "how can Wilson loudspeakers sound good when they use drivers manufactured by Wilson". "Off the shelf" or "stock" or "third party" is the term or phrase you mean to use. 
SEAS, Scanspeak, Dynaudio, etc. have invested untold millions (billions?) of dollars in R&D, tooling, testing protocol, etc. To deliver the best bang for the buck to the consumer, the logical choice is to design a loudspeaker with third party drivers spec'd to ones own requirements. When manufacturers implement their own completely proprietary drivers, you the consumer pay a huge premium for that bragging right which likely delivers little or no benefit over the famous third party suppliers. The saying "specialize in one thing and do it better than anyone else" applies here. The Scandinavians, by and large, have cornered and captured this area just like the Taiwanese have cornered the market as leaders of carbon fiber manufacturing of bicycle frames. Think of it as market-reality. 

No... OEM does NOT mean made by the producer of the finished product. It only means the company that produced the equipment originally installed in the finished product. So for Wilson, OEM drivers would be SEAS, ScanSpeak, and Focal.

Uhh, yeah it mostly does. On a Ford, OEM means a fender or bumper cover or other genuine part made by them (though the part may be made for them by a subcontractor). Completely third party parts like tires are not OEM. SEAS, ScanSpeak, and Focal are not subcontractors, they are the equivalent of Firestone, Goodyear, and Michelin. Off the shelf, stock, or third-party. 

fsonicsmith,
There are worthy exceptions, Vivid for one.

I don't follow you. Are you saying that the buyer is not paying a premium for Vivid's proprietary drivers or that the benefit of the proprietary driver is worth the premium charged? I assume you mean the latter because we all know Vivid loudspeakers are pricey. I am left to ask-how do you know that the benefit over off-the-shelf drivers is worth the tab? It seems an impossible thing to know. Only the developer could know and they will never be objective and neutral on the subject. i agree with those above that say that these alternative driver materials and design features are mostly for marketing-they create the "sizzle to the steak" that makes the buyer salivate and say, "I have got to have those!", while only conferring tiny incremental advantages or SQ improvements that could be far eclipsed by spending the dollars elsewhere. If money is of no consequence like to a Chinese billionaire, I can see going for it. For the rest of us, it makes little or no sense because for all of humankind, it's more a lateral move than a vertical one.