It's difficult to put a finger on exactly what it is, but there's a subtle difference between most paid games and most free-to-play games.
Part of it is expectation: If a customer pays $60 for a game, they expect a polished, immersive, expansive experience. On the flipside, paying nothing for a game lowers the stakes dramatically. If it's bad, the only thing you've wasted is your time.
In the case of "Fortnite," it doesn't "feel" as vacuous as so many free to play games do. There are no timers stopping you from doing certain things, only accessible by paying to speed them up or waiting hours of real time. Quite the contrary: "Fortnite" feels like a premium experience.
There's a good reason for that — "Fortnite" was in development for years, being built as a tower defense game (now known as the "Save the World" mode). The free mode that everyone is playing, "Fortnite: Battle Royale," was built on top of a long-in-development, highly polished game.
It "feels" like a premium game because it's basically a premium game.