To paraphrase "people who believe in predetermination still look both ways before they cross the street". You have free will but the timekeepers exist outside our timeline. That's the reason they can watch everything like a movie and why the Infinity stones are powerless there. There are over 14 million alternative timelines (verified by Dr Strange) where the Avengers lose. The sacred timeline is the one where a rat saves Ant-Man and the Avengers beat Thanos. So the timekeepers probably pick the sacred timeline but it's because the other timelines are pure hell. Imagine a timeline where D-Day fails or Abraham Lincoln isn't elected. Basically we're free to create our own history but most of those historical timelines aren't worth keeping.