> still I don't understand why there aren't authentic "Taquerias" (taco shops) in the US,

In most of California and the parts of the Southwest I've been to, taquerias are very common.

> The closest I ever tried was at a Taco Truck in LA, but that was just one food truck.

If you were in L.A. and you couldn't find taquerias, you weren't looking very hard (or you had a bad guide.)