Mathematica has a Mac-native frontend, and always has. They've used Qt for the Linux/Unix frontend since 6.0, not sure if they're using Qt for Windows or not.
Most of which wouldn't even run on OSX if they weren't written in a cross-platform GUI that dramatically reduces development times.
I'm sure the VLC guys would like to spend their days dealing with complicated video codecs rather than the craziness of yet-another-goddamn-GUI-platform.