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

To each their own but part of the discovery process involves selling and trading gear to get different gear. 

I have a lot of respect for the serious DIY guys and what they build as part of their hobby...but I don’t see any savings or creativity in slapping together a kit. 

Sometimes it's better to not think so much about how a speaker is made but rather to just enjoy it if you like it.  Kind of like tacos...

https://m.youtube.com/watch?v=JuBGPylPVIU

Lots of high end speakers use paper pulp woofers because they are lighter, (especially for larger drivers.) and offer better reproduction of live instruments and music.  The “paper pulp” each manufacturer has is a closely guarded secret recipie usually a composite.  As for using other brands for their tweeters and mids for example.  They usually have it built or modified to their specs.  It’s like hiring the best company for the job.  For example in the past focal made tweeters for them, then they moved onto scanspeak (from Denmark) making their tweeters to their spec.  Often even when a specific tweeter model is used the speaker manufacture will have it modified slightly or customized in some way to make it their own.

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.

OEM drivers work great but the cabinet and the crossover are an equal part of the sound.