Where in the world did you get that idea from?
I've worked on and flown in a good many WWII vintage warbirds and the vast majority are mostly original parts.
Yes, engines have been rebuilt over and over. Control systems have been overhauled, cockpits brought up to modern conditions, etc, but the majority of WWI warbirds flying today contain mostly original parts.
One reason for this is the quality of work done during that period tends to be far better than the same work today. There are, of course, many exceptions, just like buying a car built on Friday (don't!), but those bugs were worked out years ago, in combat or in lengthy use.
Incredible recoveries such as "Glacier Girl" and aircraft recovered from jungles and from underwater are going to be used largly as patterns. Built-from-scratch planes such as the Me 262s, Grumman F4Fs, etc. are worthy enterprises and should be supported. As long as originals (mostly) are available they are always going to be the more desireable option.
Tom