Because they operate on such airports: see "Operating bases" here:
https://en.wikipedia.org/wiki/Wizz_AirAlso, if you are running the current GSX version, you won't just "get Wizz air", but you'll see a menu with all eligible operators for that airport, so you can choose which one. The eligible operators are LOT, Swissport, Wizz Air and BGS.
If you don't see operator selection menu, download and install the latest GSX installer.
