>>712734065Legally required. If you call your game exclusive, it has to be exclusive, else you are guilty of false advertising. They got in trouble for that in the past.
Simultaneously, they have to do some degree of marketing for their PC release. Sony's PC releases are the significant majority of game sale revenue. Helldivers 2, for example, which is published by Sony, currently has 70,000 players online, of which 67,000 are online through Steam, meaning the other 3,000 are online through a Playstation console. Since their games sell over 20 times as many copies on PC, they can't just pretend it's not coming and actively want to market it. That is why "console exclusive" as a term has died and "only on [console] *also available on PC" came to exist.