Banks and Fees

I am probably pretty naive about how banks work, but here is my impression:

I give the bank my money.

The bank invests my money to make interest.

The bank gives me part of the interest.

The bank uses the part of the interest it keeps to cover expenses and make a profit.

If the bank can't make enough money to cover its expenses and pay some interest, they aren't very good bankers, are they? So what's the deal with charging fees? My money is moving to credit unions . . . .