Because a black hole of that mass would be tiny (smaller than a micrometer according to the article), and gravity is a very weak force. It would pass through you like through butter and bore a tiny tunnel, but it's so small it would probably leave you unharmed.
I know this is just an idiom, but it got me to thinking about just how much easier it would be for the PBH pass through you than a knife through butter. Some back of the envelope calculation gets me 40 orders of magnitude. If you compare the ease of a knife through diamond to a knife through butter, even that difference is many orders of magnitude less than a PBH through the human body.