No mass-start stage has ever gone all-out all day from the gun. There has never been a need for it, it's just not how cycling works. According to Wikipedia, "The fastest massed-start stage was in 1999 from Laval to Blois (194.5 km), won by Mario Cipollini at 50.4 km/h (31.32 mph)."
Even in running, if you're just a little bit faster than someone, think about how much easier it is to catch someone than it is to stay away from someone. To get away and stay away you have to go really hard at the beginning to get away, and then they have to maintain it. Whereas the chaser can just run whatever steady pace they like and reel them in. Cycling is like that, but more so.