There are a few reasons why I think this happens.
1. They are past their prime. They had their heyday and are now simply basking in the sun. I mean if I was someone like Ridley Scott or Stanley Kubrick and I had already done Alien, Blade Runner, Gladiator, etc. I'd just sit back, relax, and enjoy the fruits of my labor. I made a dent and left my mark on pop culture and history. I can die happy. At least that's how I see it.
2. Ties together with the notion of their prime being behind them. They no longer have as much ambition, passion, or enthusiasm for their projects like they used to. When you are young, you have the motivation, the push, the drive to strive to make something epic, memorable, and awesome. Once you have achieved that and more, you settle into this mold or routine of same old, same old, much like any other job. Something that was once new and exciting and exhilarating has now become tedious, repetitive, and boring. You no longer have the same desire to do it as you once had before.
3. The money. Some directors know that they have already made their magnum opus, or at least have a few films or so that they have great pride in having made. At this point in their lives, they simply don't care as much, and are in it probably more for the money than the passion of making a certain film. Much like a regular job itself. "I have been promoted to District Manager, I have a nice steady income, I can settle down. Even if the job is a chore, it provides my paycheck, so why bother?"
Those are my three cents, at least.