Just like... "Actions speak louder than words". This is because it's easier to say you're gonna do something than actually putting in the effort to do it.
I guess it's the difference between dreaming and then making that dream really come to pass (which would take some time in doing.) For example, "I'm gonna be a millionaire by next year." Most people wouldn't believe those words. And while it could come to pass, it's better to keep things under the hat until you're nearly halfway there. Then words will have some credit.