Phone batteries are typically smaller (less energy which can be violently dissipated) than most power banks.

Naturally you will ask, what about tablets and laptops? They are prohibited from checked luggage for this reason. Power banks however are smaller and easier to conceal.

The risk is really in a fire developing in your bag down below in cargo, where no one can see it. By the time the fire alarms go off, it's too late and good luck if you are over water or the Arctic. If it happens upstairs they can at least tend to it with a fire extinguisher or bag/blanket.

See ValuJet Flight 592, fire in an airplane's cargo hold is probably one of the scariest ways to slowly die.

It's all about corralling risk. You can't tell people they can't bring their laptops. But power banks are unnecessary nice-to-haves.