Supply vs Demand. It's not that surprising Cat/Griz is now a high-ticket item. The university knows the game is guaranteed to sell out, it's the highest-demanded ticket every two years, etc. They're going to take advantage of that and sell at a premium. If they sold those tickets for $70, the secondary market would end up still selling for $300+.
And this isn't a game for the rich. If one wants to attend the game, decisions have to be made on how are you going to save up for that game. What are you going to give up spending money on in order to attend this game? I am certainly not rich, but I did buy season tickets to avoid this very situation. In 2023, I didn't have season tickets and ended up paying close to $500 for two tickets. Decided to bite the bullet and got season tickets in 2024. A lot of us buy season tickets to avoid paying out the nose for Cat/Griz single game tickets.