Really?
There's nothing stopping Sony from allowing cross-play with Nintendo Switch except Sony. Cross-play doesn't mean that Switch and Xbox One owners have their own separate special intertube for matchmaking. Psyonix has made statements that adding it to Xbox One and Switch was as easy as changing a few simple settings in their netcode (or clicking a checkbox,
as they describe it). So if it's that simple to do for Xbox One and Switch owners, it's equally easy to do that for PS4 and Switch owners.
It's simple to decipher how to do it, too: if a Switch player starts a game, if a player on Xbox One joins the game, no PS4 players can be added to the matchmaking for that game; if a Switch player starts a game, if a player on PS4 joins the game, no Xbox One players can be added to the matchmaking. Considering that code already has to exist for the PS4 version to parse out games that PS4 users aren't eligible to match with.
So if it's that simple by a developer's own admission, there's only one reason why it hasn't happened. I'll let you guess what that reason is.
Gonna try and shout me down by saying I have no proof again? Disc Jam by High Horse Entertainment (which looks like a love-letter to the game Windjammers) is a game that has no Xbox One release to muddy the waters, just PS4, PC and Switch. And look at that, cross-play for PS4 to PC and Switch to PC, but absolutely no PS4 to Switch. So yeah, using Xbox One as a crutch for your argument is not cutting the mustard. I'll even take to Twitter and ask the developer a direct question on the subject if you're somehow not going to concede to the obvious reality that this isn't just about Xbox One and that, yes, Sony has a problem with any game played through PSN being cross-play and that, no, Switch-PS4 cross-play is not going to happen, no matter how many games without XB1 versions get released.