Not surprising really if you think about it.
Look at it strategically: Sony have a lot of cash and mindshare coming off the PS4 gen which buys them a lot of credibility with the bigger 3rd parties. They also have a rock solid first party line-up already, so its not a priority for them to pump more money into that side of things.
On the other hand although MS has shown great commitment to GamePass and securing a bunch of studios to keep a steady flow of product onto the service, its not really a setup thats going to be compelling for those bigger third-party publishers. Putting a title on GP at launch means sacrificing sales across all platforms which is a large amount of cash upfront. Yes they can bolster their income with MTX and post-sale transactions, but realistically they can get the same result by pushing a FTP/GaaS model which again would apply across all platforms.
To them its a win-win if they can take Sony's coin for limited exclusivity arrangements, and then take MS money for putting the same titles on GP further down the line if they feel like it. Its especially good as Sony really aren't concerned about anything more than console exclusivity, PC versions can come out day and date because they aren't specifically beneficial to their rivals on the green team.
By focusing on GamePass it puts Xbox in a weird position regarding buying timed exclusivity deals. They can do it without offering day #1 on GP but that weakens their pitch for that service, but if they want parity with their own product then they'll really need to pay big upfront to offset the potential loss of PS5 sales. Its a setup that works fine with smaller titles, but for AAA/big IP, not so much.