WIMP is the worst thing that ever happened to GUI. It's so clumsy.
Apple never licensed PARC, but Xerox was sold Apple stock in exchange for technical information. Microsoft was originally working on software for the Macintosh and never strictly licensed any of the GUI technologies. Windows 1.0 actually didn't support overlapping windows because Apple claimed to have IP for that, among other features. When Apple, HP and Microsoft went to court it was pretty much discovered that Apple didn't own ****. By the time Xerox went to court against Apple for their IP violations, though, the statute of limitations had passed and it was too late.
And then there's the fact that X uses a client-server model, which was "stolen" by Microsoft and Apple once commodity bitmapping hardware became fast enough to run it.
Nobody really stole anything. I know people want to hate Microsoft, but for the most part their programmers approach the problem the same way pharmaceutical researchers do. There might be competing products on the market, but if everyone is using similar TCP stacks or graphics server models or HID drivers or hotkeys and basic function it helps everybody.
And now Xerox's patents have expired and all of that delicious IP is in the public domain.
The only suits that are still around are 'look and feel' suits, but every major OS vendor, GUI toolkit and desktop environment have enough 'look and feel' property to bury anybody who tried to sue them.