Fortnite in it's current iteration, began as an afterthought to the single player Fortnight. It was thrown out there because PUBG was the IT game at the time and Epic/TenCent? thought they could quickly translate the game into a Battleground type game and then FN took off and I don't think they ever completed the Single Player game.
I remember watching Twitch/YT of some PUBG games and got bored and saw a FT video and was hooked. I never played much cause I suck at "Twitchy" type games but loved to watch.