They're just too complicated to remember, and too complicated to tie.
Agreed. Although I learnt that Jug knot, and think it's a fun knot, I've pretty much forgotten it again as it's too specialised. I frequently carry a bottle on a length of chord, and I just use a Constrictor knot or a Clove Hitch. A Constrictor knot is very secure and tight even on bottles that don't have much of a lip. I started using a Clove Hitch (on bottles with a lip) because Constrictor could be a bit inconvenient to undo.
They are both general knots, and 2 of the 5 or so that everyone should know. (The others are Figure-of-Eight, Tape knot and probably Bowline.)