Here are the messages you get when you try to log into an Xbox or a Switch, with and account that logged into PS4.
"This Fornite account is associated with a platform which does not allow it to operate on Xbox. Neither the Fortnite website or Epic Customer Service are able to change this. To Play Fortnite on Xbox, please create a new account."
"This Fornite account is associated with a platform which does not allow it to operate on Switch. Neither the Fortnite website or Epic Customer Service are able to change this. To Play Fortnite on Switch, please create a new account."
The general consensus across the entire internet is that it is Sony's fault. The only time the issue ever comes up is if someone tries using an account that has logged into a PS4 at some point. Never when dealing with any other system, PC, phone, ect.
Even if it's Sony's fault, Sony has NO POWER WHATSOEVER on what happens on a Switch or an Xbox so at best Sony asked Nintendo and Microsoft to block Fortnite accounts created on PS4 and both Nintendo and Microsoft are then accomplices to this. So worst case scenario all 3 brands are just as guilty, though Sony could be the one asking for this cause we all know Sony does not want to go for cross-platform gaming.
Saying that Sony blocks an account on Switch for Fortnite sounds as ludicrous as saying that Sony blocks Forza on Xbox should Forza not work on Xbox for some reason. I could conceive this if Fortnite was a Sony IP, but it's not so again only way to get this blocking to work is with the complicity of both Microsoft and Nintendo... And also the owners of the Fortnite IP must be accomplices.
The other way around would make more sense: Someone creating an account on Switch for fortnite would see it not work on Sony cause Sony can block anything it wants on Sony hardware, obviously but Whatever machine something is blocked on is THAT machine's fault primarily.
Congrats, you are the only person in the world that thinks Sony is not the guilty party in all of this.
I suppose you also believe Sony when they say they don't like Crossplatform, because they need to protect the kids from the evil people on other systems.
Here are the facts:
Fortnite is crossplay between PS4, PC and Mobilie.
Fortnite is Crossplay between Xbox, PC, Switch and Mobile.
You can log into your epic account on the Xbox, PC, Switch, Mobile and get all your items, progress, ect with no problem. If you have ever logged into your Epic account on a PS$, you can no longer ever use that account on a Xbox or Switch.
Xbox, Switch and PC can all crosssplay with Minecraft and Rocket League. PS4 is PC only.
If you somehow can read all of that and believe that somehow it's Nintendo or Xbox's fault, then...