My thought is that even when the tire is brand new...the tread dosent account for 100% of the value of the tire.
So lets says hypothetically speaking and using nice round numbers for the sake of my sick nasty awesome math skills...
Tire = 100% new = $100
Tread = 25% of value becasue other factors contribute to tire value. (Side walls, construction, rubber compound etc.)
So, even if the tire had ZERO tread left, the tire would still cost $75.
Its a crude measurement and blunt but I think it shows what Im trying to say.