What prevents DIY speakers to become the ultimate?

I'm interested in making my own speakers as well. I'm not interested in engineering my own- nor am I interested in re-inventing the wheel. I understand that the Thor is a solid design, are there others? Is there a short list of Best DIY kits or designs.

Northcreek notwithstanding, a non-dedicated non-OEM can't buy tight-production tolerance drivers required for superb-imaging, ultranatural response. As alluded above, the best manufactures have custom-made drivers specced within a tight sensitivity window, and even then sometimes pair-match them. (Verity even jeeps a response curve on each midrange for replacement if needed!)
It's a given that Madisound, Solen, etc, get the outer 2/3 or so of a given driver run! I've built a couple of 2 and 3-ways, and once I heard a midrange voicing change of only 1/3dB over 1.5 octaves during a crossover massage, I gave up, knowing that I'd NEVER be able to afford drivers that well matched! This is by far the largest impediment to DIY
ULTRA-high quality production. Though a reseller can sit there and wade through a box of drivers, sorting their sensitivity with a Radio Shack SPL meter, and then selling so-called "matched pairs", that's only going a short way to providing yje standardization required, 'cause you gotta do it with the crossovers next!
Tweeters and caps are notoriously wayward in spectral response. When I heard Verity Audio's midrange, I cried, and now have a couple crates of nice big inductors, caps, resistors, etc., if anyone wants 'em for a dime on the dollar! Spendor, Revel, Snell, and even Boston (!) do a VERY creditable job of pair-matching and building to a target reference with decent precision. Who's going to build us DIY'er some clones to work with? I'll stick to power cords and other non-transducer components....
My hatsoff to those still trying!

Although tempting,I passed on DIY kits...it is hard to justify spending $500-600 on a pair of DIY monitors when you can buy a very good pair of used monitors for that kind of cash...such as the SPendor 3/5,QUad 11L,etc...and be certain of the outcome.. today even $500-600 new will still get you very good sound... w/ alot less headaches...

I spent about $1400 on a pair of DIY speakers, and am extremely pleased with the outcome. I built a set of Voigt Pipe cabinets from oak, and loaded in a set of Lowther EX3 single drivers. The sound is exactly what I was looking for, and the job was quite easy. Didn't have to worry about building a crossover, because I didn't need or want a crossover in my speaker system. Just one, fullrange driver per channel, with point source radiation. No problems, very happy, saved money.