Ah, took me a bit to figure out the perceived issue. The general rule is you do not include the preposition if it is superfluous to the meaning.

So, is there a reason that the "at" may not be superfluous? It seems one looseness in language leads to the need for specificity elsewhere. "Where were you?" is often treated as if it says "What were you doing?"

I ask: "Where were you?"
I could get:
"Working on the car."
or
"In the garage."

I ask: "Where were you at?"
I would expect only to get:
"In the garage."