They don't wanna give Xbox and Switch access to the much bigger PS4 userbase (therefore indirectly improving Xbox and Switch online experience by reducing queue time), and they definitely want people to be forced to buy a PS4 if they want to play with their PS4 friends. The PR was so bad with Fortnite and Rocket League that they allowed it only for these games hiding behind the "beta" bullshit.
I think you pretty much hit the mail on the head. Right now, Sony is basically only accommodating rare titles that have basically "moved the needle" in the overall gaming meta, so they caved on Fortnite, and Rocket League is soon coming. Maybe if Activision decided to be a real pain in the ass and made angry statements about crossplay for Call of Duty, or EA did the same with Battlefield, Apex, Anthem, Sports games, or 2k with NBA 2k) maybe things would happen with those titles as well. But those big devs probably don't care, they got a shit ton of people playing on any platform, and would rather not challenge Sony or any of their platform partners they depend on to sell the multi millions needed to be profitable. Bethesda tried to talk a little smack with Fallout 76, (and they previously butted heads with Sony before on mods being available on Fallout 4/Skyrim but that game is such a piece of shit and pretty much no one cares now, so no action there.
But if you are companies similar to Chucklefish or say somone like Hi-Rez (Paladins, Smite, Realm Royale)- both smallish crews who are pushing for crossplay and implementing it in their games with smaller, but dedicated communities (with Sony left out of course), they will get no luck because Sony just don't care about the small fry.