1. They Have a Full Time Job
Most of the time your project customer is a sponsor for the project because it is something that falls in the scope of his normal responsibilities at his organisation. Yes, sometimes it's all about everything he does and overseeing your project and your team is 100% of what he is doing. But usually that is not the case. Usually he's acquired the project because it was assigned to him or because it is important to him but he still has 90% of his time taken up by his normal tasks that he performs on a daily basis.