Short answer... because its anti-historical. If preserving historical artifacts is what we are about, we can't condone the vandalizing of them. Say what you will about what a gun once looked like... we don't and can't know...we've never seen it and we can't return it to its "original" condition. We can remove some old parts that were added during its period of active use and replace them with new ones ... how this constitutes an improvement in a historical sense escapes me. How about the cute trick of dealing with cap corrosion by shortening the barrel from the breech, moving the pin lugs and shortening the stock?... done on high end Pennsylvania rifles. How in the world can changing a rifle that much be called "restoration." Arguably one could make a more convincing case for military arms but it is just as anti-historical. Most "restorations" of early Land Pattern Muskets are wrong no matter how well done the work is done, simply because much less was known about very subtle changes that had not yet been recognized.
The article Dr. Tim Boone posted is very interesting. The observation that cars are about the only "collected" item where restoration is not a detriment is spot on. I've had antique cars on an off since the 70s... a 26 Cadillac, 10 and 11 Reos, a 29 and a 34 Rolls-Royce etc... I am currently rebuilding a 10 Mitchell. I've always had a problem with the "looks new is better" and the "cosmetics are everything" school of car restoration. Its refreshing to see that this point of view is being widely challenged... Unlike antique arms, there really isn't any such thing as static storage for a car. They go bad just standing in the garage, so some constant maintenance is always required. And, unlike guns, most are expected to actually work... we can shoot old guns (I do occasionally) but being shootable is rarely a major criteria in serious collecting.
Running is a major criteria in car collecting and if you're going on a public road, its a good idea to give some thought to safety issues that didn't exist 60 or 80 or 100 years ago. I'm putting 1913 vintage electric lights on my 1910 car because I've been stuck on the road, after dark, with nothing but kerosene side and tail lights. This is a detail that would cost "points" if I had the slightest interest in competitive judging... In fact, none of my cars would ever have won a trophy at a car show... At best its problematical how much restoration enhances value when its also well known and accepted that its practically impossible to recoup restoration costs when a car is sold.