The key factor to Provenance's business model is that they aim to deal only in the finest warbird restorations. Possessing the best of the best puts them in the unique position of being able to define the upper edge of the market envelope, and that's prime territory when you're looking to maximize your profit margin. If you've got the means and the vision to do it, why not?
One thing that higher value brings to these planes is an increased concern for their well-being and preservation on the part of the investor. You can knock the "rising cost of ownership", but it really is a prime factor in securing the condition of the planes for future generations. There are plenty of historically important warbirds out there right now who's value hasn't escalated considerably, and those are still falling by the wayside and/or being scrapped (amazingly) as a result. I'll probably never be able to own a Corsair or Mustang at this point, but there is a whole new crop of "future unobtanium" out there begging for a new home, but still finding its way to the scrapper.