It has all to do with the collector's market, and what the people vote for with their $$$. Something that's rare, and in half-decent original condition is almost always going to command a large premium that a repaint just isn't going to. Of course, if you're getting the bicycle without consideration of what it might re-sell for (which some of us can honestly say is the case with their own bikes), there's no one stopping you from repainting such a bike (although there are some painters who will advise you not to to ease their conscience before they will go ahead and do it.

That said, I don't know of anyone who would object to cleaning up the weathering on the chrome - which accounts for an awful lot of this bike's "old and tired" appearance. The paint actually looks astoundingly good given the bike's age.