Due to the way the universe expends, even if you travel at the speed of light forever, you can only reach a finite portion of it. The longer you wait, the less that is. Because of this, an AI that doesn't send out probes as fast as possible and, to a lesser extent, as soon as possible, will only be able to control a smaller portion of the universe. If you have any preferences about what happens in the rest of the universe, you'd want to leave early.

Also, as Oscar said, you don't want the resources you can easily reach to go to waste while you're putting off using them.