Game Pass has never been about a "full throttle" strategy that "opens it up to any and everybody". The service is exclusive to Microsoft Windows and Microsoft Xbox.
You misunderstand the nature of that service. They're willing to open it up to Sony's games if Sony would desire to monetize their games on Microsoft's platform. That would open up Sony games to a much larger user base. Sony doesn't because they calculate this will cost them console sales, and ultimately a larger customer base buying software-- theirs or others-- on a market they control. They'd rather control the market where they command a larger share of revenue.
For the same reason, Microsoft would maintain a Game Pass exclusive wall against Sony, Nintendo, and other competitors by locking those marketplaces out.