Because getting 60 bucks once or twice a year is less than getting 10-15 bucks per month every year. If you feel your service offers a heightened quality on such a level that this isn't suitable, you can always hike up the price a couple bucks. I hope you don't really expect me to explain to you in detail the math of why long term subscription models are more attractive to every branch of the entertainment industry than one time purchases here and there.
You're incredibly naive if you think GP is going away if/when MS has a stronger market position, or that Sony isn't going to be making a concerted effort to move in the same direction in the future.
So let's compare a made up AAA game.
Game pass has 10 Million users. Source: https://www.theverge.com/2020/4/29/21242004/xbox-game-pass-10-million-subscribers-microsoft-q3-2020-earnings
Let's say 70% is paying full price of $10 a month. Many users are using the service through $1 deals, etc. This is $70,000,000 per month. so $840,000,000 in a year.
Let's say this Sony sells 10 million copies in a year at $60 a copy. That's $600,000,000.
So it looks like Microsoft make more money right? Wrong.
Sony make another AAA game in the same year and that's ANOTHER $600,000,000. So now we have $1,200,000,000. Microsoft however are still going to earn $840,000,000 in the year as both their AAA games are included in one price.