Because there's this belief that a for-profit company will naturally be more efficient. No idea if that's actually true in practice though. Or if efficiency > the profits the company takes.

I don't know of any large community ran utilities, just small ones. I'm guessing the scale starts being a problem eventually.

They can be efficient, but I ly if the incentives align towards the desired definition of efficiency. If you give a company a natural monopoly and protection from competition. Then it's most efficient way to make money is just to raise rates

TVA and the NY power authority are genuinely massive, government run utilities. Both are also known for pretty low power bills.