It's actually because logging in to a Fortnite account on PS4 irreversibly locks that account to Sony's walled garden.
I created my Fortnite account on PC and played there. Then I logged in on PS4, and played there. Now when I try to log in on Switch, I'm not able to. Here's the actual error message given:
"This Fortnite account is associated with a platform which does not allow it to operate on Switch. Neither the Fortnite website nor Epic Customer Service are able to change this. To play Fortnite on Switch, please create a new account."
The problem is that even disassociating the account from Sony won't then allow me to use my Fortnite account outside of Sony's walled garden. The account is permanently tainted by PlayStation.
Most importantly, the irreversibility of Sony's taint upon your account isn't disclosed until after the fact. It's a classic anti-user misdeed.
Anyway, Hacker News seems like the right place to ponder SaaSS user account foibles.