Calligraphy vs Pen inks

Noob question here: why can’t calligraphy inks be used in fountain pens? Is it a consistency issue? Not intense enough? Potential blockage problems? Just trying to understand the mechanics of it all. Thanks in advance.

Just bought a tone of Ferris Wheel Press inks and I’m in love!

PS. One more question : How do you use fountain pens on your Smithsonian notebooks? The Nile blue paper is lovely but so thin, the ink goes right through!