The features offered by commercial engines are lightyears ahead of FOSS engines, especially at the high end.
If you're running a commercial game studio in such a brutally competitive market as video games, how can you afford to use FOSS? Your choices are go without high-end features that your competitors do have; or divert resources from your game content to build them in-house. Both seem less attractive than "just buy a commercial engine" to many studios.