Because Sundar Pichai has a strong public track record. Every CEO ends up with warts, but I have some sense of what he's about. The leaker, I have zero information on. Given known vs unknown, I put more faith in the known. Whether I by default believe or disbelieve depends on who's saying what.
Sundar Pichai's claim to fame was getting the Google Toolbar installer (and later the Chrome one) injected into the Adobe Reader installer. 
Don’t worry everyone it’s different now!