Because unlike Microsoft Sony doesn't believe in just giving their stuff away. They will make some money on it first then they'll put in on PS+. It's the same reason why they not out here making a subscription service for their movie and TV content. You're going to either buy or rent Across the Spider-Verse or they will license it to Netflix. That's their company strategy in general.
Well its not sound because they are making less money than Microsoft and Nintendo but thats their right. Jawn aint worth 40 bucks