Part of this is also just the options that first-rate actors open up for you as a writer or director that less capable ones cannot. If you think of the performer's talent as kind of a box that you can fit your narrative and emotional depth in, you just wouldn't try to ship something unless you have a box big enough to hold it.
If you have someone like Anthony Hopkins or Ian McKellan on hand you can give them long, baroque speeches and they will nail it. With a less capable actor you would be forced to keep it simpler because most of that stuff might sound corny as hell in less capable hands.
1. Luck does indeed play at big role in getting a break, the right roles, the right director, etc. A lot of people who could have become big stars don't. People know this and leap from there to the whole thing being pretty random.
2. It's often not obvious what makes a great actor that much greater than someone who is not quite so great. Film probably accentuates the differences. But even with mid- to top-level professional theater, the whole cast is probably pretty solid, but the stars really shine in hard to put your finger on it ways. In more "normal" professional roles, it's usually a lot easier to peg why someone is just better than someone else.