Yea i don't get it either but i think it's because most gamers like their games new and day 1 with everybody else to play with everyone else immediately. So ppl use their 20% discounts pretty frequently and it looks like the best deal to them, and for their immediate situation it probably is.
I don't really game like that online and i have plenty of backlogged games that i don't need the latest and greatest games day 1. Me, i do not mind waiting and buying pre-owned. I get most all my games from Gamestop when they have their B2G1 which i feel gives the best value. 20% off a brand new day 1 $60 game is still $48. IF you have/get pre-order bonus coupon for an $10 that takes your savings down to $38 but you typically don't get that with every game i don't think. With Gamestop i can pair three of the latest released pre-owned games for $104($52 times 2) which is a value of $35 each.
And i get that they give super low shyt amounts for trade-ins,but that's how they make money off pre-owned games. I typically sell/trade games thru local Facebook groups these days so i don't worry about trade-in values anyway. So yea i don't get the hate, different strokes for different folks.