Darcie,
Tapioca starch is made by washing the starch out of the cassava, and then dehydrating. It's pure starch. Flour is made from dried, ground cassava root, so it contains fiber and other solid elements.
If you use starch to try to make pao de quejo, then you have no solids in your dough. You can add a LOT of tapioca starch to milk before you get a workable dough, like liters of it. Believe me, I know.
