Because it's the most exhaustive blend of games from Western developers and Japanese developers. Microsoft is typically lacking in Japanese support, and Nintendo lacks a lot of AAA third-party games (Japanese or otherwise).
First-party games do help. Gran Turismo, God of War, Uncharted, The Last of Us, etc. But they are merely a contributing factor and not as essential to Sony as they are for Nintendo.
Lifetime Sales Predictions
Switch: 144 million (was 73, then 96, then 113 million, then 125 million)
PS5: 105 million Xbox Series S/X: 60 million
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima