I mean, yes, obviously having trainers helps, but really the basics of a good exercise plan are pretty readily available. As for equipment, really most gyms have enough equipment to get 95% of what most actors achieve.
And yes, it's part of their livelihood, but they can't actually spend every waking hour on training. Both because it's actually detrimental to gaining more muscle, but also because actors actually do have other things to do, like acting in other things, learning lines, appearances, etc.
This is not to take away from their hard work and sacrifices w.r.t. diet. Obviously they have to work very hard at it. But the actual reason most people won't end up "looking like a super-hero", even if they can devote lots of time to training and dieting, is because of the steroids.
(Lots of people do take steroids, btw.)