The problem with bells is that they aren't very directional. It's hard for my brain to figure out from which direction the sound is coming from. Someone speaking "on your left" is much more directional, and it includes important context as to what the warning is about.
Its pretty safe to assume on a trail if you hear a bell that a bike is coming up behind you.
Or from the side or oncoming and he's just behind the crowd of pedestrians ahead of you.
Yes haha - a bike coming from _somewhere_