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

Because there is more to the sound of a speaker than just what the drivers are made of. Like crossover components and the wire used to connect it all together. Or cabinet design and resonance. That said, many classic speakers like JBL used pretty good materials in their construction (the paper cones were rumored to use US currency paper, and massive edge wound COPPER speaker coils). I have had JBLs in the late 70's and early 80's that were advanced technology at the time, like the 066 dome radiators that used vapor deposited aluminum for stiffness. Yeah, they are not as sophisticated as the diamond tweeters B&W uses, but sonicly I am not sure the diamond tweeters are a real improvement. I have B&W 803Ds currently and have made significant sound improvements by modding the crossovers (particularly the caps and resistors)  and adding heavy brass footers and solid maple base plates. I recently rebuilt some JBL L212s from the late 70's, and after a complete upgrade of all crossover components and re-surrounding drivers, an A/B listening comparison with my modded 803Ds left me wondering how these 70's era speakers could sound so good in comparison to the B&Ws. It just goes to show that technology for technology sake does not make a great sounding speaker. Marketing departments can have a big influence on how technology is pushed to the consumer, rather than common sense engineering.

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. 

fsonicsmith,
There are worthy exceptions, Vivid for one.

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.