Because they don't know the words??
Ok, I know, bad joke.

I'm just guessing here.
Transformers hum from the power running through them I believe.
It has something to do with resonance from the inductance.
Under a large load a lot of power flows through a transformer.
This makes it hum. The sound is carried down the lines.
Like big guitar strings.

Pooh Bear

PoohBeah gets the ceegar. It's the resonance in the lines. The sound emminates from the transformer windings and is propagated down the lines. It can also occur do to leakage across the insulators, but with that you usually hear a sizzling noise too.

Cool, I got it rite.

Pooh Bear