I buy old cart games mostly because of nostalgia, but also because I like this style of game (referring to side scrolling, etc).
But at the same time you may be right. I wouldn't rush to buy old "rare" game IF it would always have been in digital format or still in physical production. I guess that's human nature, like buying music albums of an artist you didn't care for years but since he died, you want to fill in that emptiness he leaves behind.
Also, being physical items, they use and break overtime, making them rarer everyday. You never know when is your last chance to get it!