Some of it is just them getting hard to find for things that were made in smaller numbers. As time goes on there are less of them floating around.
But also: grading and internet hype has drawn the eyes of a certain class of investor that want to sit on these "assets." Then everyone that sees a graded copy of a game sell for a high dollar amount assumes their grungy copy with no box and their name written on the cartridge is worth the same amount. You see the same thing across a lot of collectibles hobbies, unfortunately.