Because you've got plenty of old games here (and many indies too) whose original price is 5 or 10$ and often are sold at a 75% discount (or sometimes even more).
Space Quest games are currently 75% off for me. Plus Sierra *Quest games are sold in collections, so you get between 2 and 5 games for each purchase. Honestly, they are not expensive at all even without discounts, given how good they are (if you don't mind the interface and low-res graphics).
I already have the entire Sierra classics collection (King, Space, Police, Larry, For Glory, etc). What is strange is that some of these games - like Space Quest or Quest for Glory - always had good discounts since they first got in the store. While other games (like the King's Quest series) never had as good discounts. I suppose that could also related to the game fame. King's Quest probably had more fame than the Space series. I don't remember Police Quest series ever getting a 75% discount, but I could very well be mistaken. Which is strange because it's a lesser known series than the other Sierra games.
I can understand that some people will think I'm a miser or something. Although I'm hardly one, since I already own more than a 1000 games on GOG and I support the store and publishers whenever possible.
Although I admit that some things still shock me. Like Activision selling very old (and some of them not so good) adventure games for as much as 9$, while the also "greedy" EA selling the great Ultima RPGs (with dozens of hours of gameplay and some of them still highly playable today) at only 5$. I mean... just recently I finished Ultima Underworld for the 3rd time and just this game alone would easily be worth 9$. Yet EA sells both 1+2 for 5$ ! And when there are sales these classics usually get a 75% discount.
Of course, all my arguments can be labeled as subjective or controversial. Again, my gripe is not against the price of the games "per se". I can understand a small publisher like Ziggurat or Nightdive selling old games for a high price in order to get some needed cash flow. But a colossus like Activision?? :(