The strategy is indeed deliberate and it comes from a risk/reward evaluation. Keep in mind that the goal of each serve is not simply to have it be in, but to win a point. Just as the server has been practicing the serve their whole life, the other person has practiced returning serves their whole life. Pro tennis players are good enough, I'm sure, to get practically every serve in; but when that's the goal, the risk becomes having easily returnable serves wherein the opponent gets the upper hand. You could look at first serves as the server pushing the boundaries--they want to maximize the possible reward of an ace or a poorly returned serve and in doing so they risk a lower success rates. On second serves, less of a chance is taken, they knowingly reduce the effectiveness (reward) of the serve in order to minimize the risk (double fault and giving up point). It's not that they can't control their serve, it's just that they're aiming for the very best serve they're capable of.
You could liken it to a professional soccer player taking a penalty kick. Sometimes the kick ends up sailing over the goal or to one of the sides. It's not because the player isn't any good--they could hit the ball into the net 100% of the time practicing by themself--but the fact that a goalie is trying to stop him causes the player to push the limits of his skill and sometimes fail at it.