Because you are clearly wrong.

Forgive me for being blunt, I just think it’s a bit obvious: they have grown through means other than someone going to google.com and typing “Pinterest.”

I can think of a dozen reasons why that would be the case. Google Trends does not match 1:1 popularity, usage, or growth of all products.

Personally, I’m only a casual Pinterest user and have no stake in this, but the service has gotten a lot better over the last couple years.