I can think of several reasons.
Our beloved Impalas are and always have been a niche market and that niche grows smaller every year.
And maybe, like me, some are fed up with the "Factory original is the only way." attitude this forum has morphed into; The "I can be more anal than you (How much overspray under the dash?)." attitude.
Bulletin: Late 60's Impalas are not Shelby Mustangs or MoPar Hemis or big block Camaros and will never appreciate in value like them.
And Nick, although you may have a hundred grand spent (not invested) on your '69 L72, you'll be lucky to get $40K for it on its best day.