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Simple, kids grow up, make money and buy the things they wanted when they were kids. Another thing is that once the price reaches a certain level, people begin to start doing things like speculate on it continuing to rise while some people end up thinking that because its expensive its somehow more desirable. If you tell people that a game costs 10 dollars, they'll just think its cheap trash. Tell someone it costs 1000 dollars and the price tag alone makes them think more highly of them.
Personally I'm happy I managed to collect most of my games before the markets started being retarded. Bought Doom for like 20 bucks probably 15 years ago, probably goes for above 400 now.