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

It’s the cabinet.  They use excellent drivers but the cabinet makes a massive difference.  By having such a well damped cabinet, they deliver an amazing sound.  The molds for those cabinets are pricey which is why Wilson’s are soo expensive.  

Without the cabinet, any DIY speaker will sound inferior.  You could build a Baltic Birch cabinet to the exact dimensions of a Wilson Speaker, take the drivers and crossover out of a pair of Wilson’s and put them in your cabinet and it won’t sound the same.  Very simply, MDF, Baltic Birch or Hardwoods will not dampen sound even remotely as well as cast phenolic resin.

i have personally not done testing with cast phenolic resin, but I have tested 6 different cabinet materials and each has a decidedly different sonic character.  My test was to build the same cabinet internal spec with the same drivers, crossover, port, polyfill etc.  and the end result was a radically different sound profile.  

I tested MDF, Baltic Birch Plywood, Laminated Bamboo, Carbon Fiber Reinforced ABS, Fiberglass over a nomex core and Carbon Fiber over a nomex core.  not shockingly the ultra rigid and well damped composites significantly outperformed the wood products.  Plastic (ABS) even with Carbon Fiber reinforcement was significantly worse.  

It’s the cabinet.  They use excellent drivers but the cabinet makes a massive difference.  By having such a well damped cabinet, they deliver an amazing sound.  The molds for those cabinets are pricey which is why Wilson’s are soo expensive.  

Yep.

I've had plenty of chances of hearing speakers in baltic birch or MDF, used for getting the design nailed down, then the same speaker in a CLD cabinet of the same dimensions. No other differences in the speakers. 

The improvements are not trivial. 

Lucky for the DIY community, there are some very effective, and not to difficult to deploy, constrained layer damping solutions. Maybe not up to what the high end manufacturers are using, such as Wilson or Von Schweikert, but much better than plain MDF of birch ply.