Shooting stills in bright sunlight with fast glass with super wide apertures is fairly easy. You just knock down your ISO and speed up your shutter until you have your exposure under control. But when you’re shooting video, that’s not really an option if you want that cinematic look. Your shutter speed’s generally locked at 1/50th of a second for 24p video (1/100th for 50fps, 1/125th for 60fps, etc).
It’s a bit like working outdoors in bright conditions with flash that doesn’t support high speed sync. Once you hit that sync speed your shutter simply won’t go any faster. The go to option in both cases is neutral density. Filters that go over your lens to reduce the amount of light entering it and hitting your sensor. This video from weird lens guru, Mathieu Stern, shows a slightly different approach.