Only a fool would invest 1.5 billion to make 10 mil/yr.
You ignore the appreciation in the team value. Sterling bought the Clippers for $12.5 million. Assuming for the sake of argument your $1.5 billion figure is accurate then he stands to make quite a profit from that investment. As will anyone who buys the team from him.
Besides, it isn't the money it's the status of being a pro sports team owner that is the real attraction. With the Clippers Sterling is a sports mogul. Without them he's a slumlord.