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

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,
Yes, I am saying that in the case of Vivid, the premium paid for their in-house designed drivers is worth the premium. They have pushed the boundaries in driver design, and it has paid off in pure and accurate reproduction. I don't own them, but I have heard them and I respect their achievement.

After about 40 years of audio experience I tend to judge the speaker by the final result which consists of two things important to me: sound and looks.
The designers are making a cake using the ingredients that will achieve their desired results.
Daryl(?) Wilson created the Yvette’s I own(and love) with his own particular vision for the final results. I have zero interest in the drivers he used to create this stunning loudspeaker. I’ve had plenty of exotic tweeters in house and none have exceeded the smoothness and detail of the silk dome tweeter he uses to achieve his goals. I’m pretty sure that it’s not particularly expensive but I trust that it’s the one he wanted.
 I have no particular feeling towards Wilson one way or another but after owning a lot of expensive speakers I’ve never once thought of the Yvette as anything other than a reasonable value in the inherently overpriced speaker market. I bought a package of design details that make the whole and I’m elated with the sound and worry very little about the ingredients that created this superlative product. Just my opinion of course!