This is actually a very good point.
When pants deteriorate, they lose value because no one else wants to pay as much for deteriorated pants. We look at them and go "hmmm, they look like crap, I do not want to purchase them" and thus our preception of their value goes down.
When a game becomes old, it too deteriorates. We look at them and go "hmmm, they look like crap, I do not want to purchase them" and thus our preception of their value goes down.
The same mechanic is at work here, make sure not to confuse physical condition directly with monetery value.
To be more precise, the issue is the lack of incentive to buy from the distribution service instead of a used copy. For most products you lose something when you buy it from another person instead of the retailer. Digitally distributed software on the other hand, buying an already existing license gets you the exactly same thing as buying an original copy. It doesn't matter what the "intrinsic value" or whatever you want to call it reduces over the time, as it reduces the same amount from both the used and new copies. A better metaphor for this kind of appreciation could possibly be something going out of fashion.
If I buy a chair from my friend, I get a chair. Same as if I buy from a shop. If I buy a DVD from a used DVD shop, I get a DVD. Same as if I bought from a new-DVD shop.
I mean sure, you can buy a second hand POS broken down chair that's missing a leg, or a scratched up DVD that doesn't play. But that's just as likely as buying a DVD that's been kept by a fastidious neat freak and is in perfect condition, or a chair kept by a carpenter who has repaired any damage with perfect accuracy to the original chair. You'll pay more for a chair with all four legs in mint condition than a broken unusable chair, but still not as much as buying one new.
I'll agree that there's a chance that you can buy something second hand and lose something, which isn't present with digital media, but it's not a garuantee. Between me and my immediate friends we've probobly bought hundreds of second hand DVDs from a highstreet shop in the UK called CEX. I think one of them didn't play. The rest were all basically the same as buying a brand new DVD.
There's definatly a lack of incentive though, because second hand things are generally cheaper. This is because, regardless of the quality of the actual item, not being the first person to own something has a psychological component that makes people think it's worth slightly less (there's probobly a whole lot more to it than that, like generally second hand items are sold at a point where the particular item is out of fasion/date/etc, but you see my point). But this isn't a bad thing. It's infact one of the basic tenents of capitalism. I mean why buy a new chair when you can buy one brand new? You don't see furnature manufacturers complaining because people are buying their chairs for 100 pounds and then selling them on for 70. Infact most industries deal with problems like this, but for some reason the video games industry has decided that it shouldn't have to deal with buisness problems that have faced every other salesman since the dawn of time.
If you walk into any video game shop you'll generally see huge sections of the shelving taken up by used games. My local game has way more used PS3 games than new, and they're generally far far cheaper, with a difference of £20 easily. They also usaully have buy 3 get one free offers, or 2 for the price of 1 liberally applied. But people still buy new games. The incentive is to buy used, you get more games for the same amount of money, you know what games are good because they've been out a while and the reviews are plentifull, and they'll probobly have all the annoying launch day bugs patched out, DLC (if thats you thing) will be ready and waiting on your online store of choice. But the thing is... video games are entertainment. They aren't something you purchase primarily based on value, but on personal preference. Someone who is a huge FIFA fan isn't going to go on release day for FIFA 2013 and buy last years game because it's £30 cheaper. It's the same reason why people who are into movies pay £10 to sit in a crowded theater and listen to people talking over the movie to see the latest blockbuster on release, instead of waiting a few months and buying the DVD for £10 so they can watch it whenever they want in the comfort of their own home. Entertainment doesn't really follow the same rules as necessities or furnature or clothing (well it can I guess if that's your hobby).