The quality of Game Pass thrives on the sales success or expectations that come mainly from Playstation and PC. Microsoft probably doesn't pay THAT much because 3rd parties don't expect their mid-tier games to sell all that well on Xbox in the first place (may as well explore something different), and PC + Playstation sales have them covered. Sony would have to pay more if they wish to match; it's the exact opposite situation to securing exclusives, in which it's MS that has to pay more.
Those who are actually buying these games on PC, Playstation and Switch are helping make Game Pass a compelling service. It's pretty clever how Microsoft is turning their weakness into a strength but I still am curious how much profit they're making off gaming. What they need now is first party quality to kill the concerns about "Game Pass limiting budgets to remain sustainable."