It's because billionaires are planning on living forever.
When you are going to live forever you won't be able to keep it a secret.
Controlling 10 billion people is much different than trying to control 30 billion.
When illness and death is no longer a concern for you, your only concern will be dying at the hands of another person.
Forever is a long time.
They want the whole planet.
Maybe their system of control doesn't scale well. We are social beings with limits - a single individual can only control so many others "beneath" them. Many levels of control are needed.