From experience, an acoustic guitar is about as powerful volume-wise as 1/2 to 1 watt electric guitar amp with a moderately efficient speaker. And there just isn't a whole lot of energy transmitted from a guitar pick to a string. Whatever the efficiency is, there aren't wide variations in acoustic guitar volume that can't be explained by the attack/sustain tradeoff.
Bowed instruments get around the attack/sustain tradeoff by having continuous energy input, so the attack strength can be sustained for the duration of a note.
My cello is particularly resonant, and in hindsight (is this confirmation bias talking?) has very little sustain on pizzicato (plucking) compared to my cheap, less-resonant student cello.
