Why the launcher doesn't detects already installed game?

I’ve transferred the entire Fortnite file from one PC to another. But the launcher still says to install it. Why is this so? and what is the solution to run it successfully? I’ve tried many ways to do it as suggested by people, by starting the installation, pause it and add your own file, but it didn’t work at all the installation resumed continuing for a while until I decided to stop it. Please! I really need help with a working solution.