Weird, Sony can cross play with PC. Wonder why they're opposed to Switch and Xbox.
Hopefully this changes soon.
Because Sony have nothing to gain, if Microsoft was leading they would be the ones stopping cross-platform.
As the market leaders Sony have a sales advantage whereby people will buy a PlayStation just so they can play with their friends who also have a PlayStation (the majority). If there is cross-platform play Sony lose that advantage, and someone with friends who all own ps4 can still buy a different console.
It's not good for consumers, but no one should fool themselves into thinking Microsoft wouldn't be doing the exact same thing in their position. Last-Gen microsoft didn't even allow cross-platform with 360 > PC afaik, while PS3 had Portal 2 / FFXIV etc.
LTD: PS4 - 125m, Switch - 110m, XBO - 51m
2020: PS4 - 9m, Switch - 22.5m, XBO - 2.5m, PS5 - 4.5m, XBX - 2.8m