The hero has to deal with the negative ramifications of the happenstance, and so the 'lucky break' is masked. There are exceptions, of course, to these concerns about coincidence. One group perhaps should be noted. There are films where coincidence almost becomes a character, part of the conceit of the story. Instead of hiding the odd and ironic flukes and coincidences, the plot revels in them.
Fate takes a heavy hand in the course of things, often to comic effect.
But for the most part, screenwriters go to great lengths to minimize reality-harming coincidences, or avoid them altogether. To illustrate, let's do a little model. Time allows the characters to take action according to how they've been designed, and voila, the story happens. So, with the passage of time, let's see how our 'initial conditions' evolve for our little mini-drama. You get the idea.