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. 

@fsonicsmith 
How ironic. I know for a fact that none of the parts you named for a Ford are made by Ford. The fenders are made by New Center Stamping and the bumpers by Norplass. How about Denso AC compressors? Those aren't bespoke for any car. They're certainly OEM equipment. The part being bespoke for the product has nothing to do with it's OEM status. 

Last I checked Wilson modified tweeters themselves, or at least they do for some of them. 
I like building, restoring, and modifying my electronics, but I really don't think it's possible to build a speaker on a $5000 that touches what a $5000 commercial speaker can do. To do some of the more complicated measurements on my electronics I'd need about $1000 in gear. That's not too bad. To actually design and build a speaker I'd consider worth listening to I'd need a whole workshop full of power tools and a pile of mics and measurement gear that would totally blow my budget. Anything else would be guess work. I can build an amp, and as long as the parts are of decent quality and properly matched, the thing is going to make the numbers it's designed to. A speaker can sound very different depending on if or what you laminate the cabinet with. Focal's kit designs we're like that. 
But let's say you designed a techically competent speaker. Crossovers aren't cheap and I'd spend myself into poverty just trying to voice a pair of speakers. Good caps and inductors aren't cheap.

There seems to be some assumption that a material is just a material regardless of it's manufacture. Correctly manufactured paper has a very high speed of sound while still achieving good damping and stiffness. Wilson used to use a bunch of Focal drivers which are about as far as you can get from paper. Those woven B&W drivers have great damping, but aren't very stiff and they have a rather slow speed of sound. Carbon fiber drivers are extremely stiff and controlling their ringing take carefully consideration. I'm not surprised to see Focal putting flax cones in $10,000 speakers. They have some of the best characteristics of paper and they're stiffer and lighter. Again, old school organic cellulose fibers, just manufactured better. 

Wilson seemed to love stuffing big Focal W cone woofers in things for a while. All the Alexandria line sports W cone woofers. I'm not terribly surprised Wilson steered clear of Focal mid-range drivers. Focal likes to make mid-range cones rather large and Dave strongly believes in using the smallest driver that will get the job done. One of the reasons I've heard he moved away from Focal tweeters was he couldn't get the to do much below 2000Hz. I have to wonder if part of the reason Wilson moved to paper woofers was because they couldn't get Focal drivers anymore.