I guess they do this because someone figured out they could. I'm sure once upon a time there was a bank that only took deposits and just stored your cash in a

I guess they do this because someone figured out they could. I'm sure once upon a time there was a bank that only took deposits and just stored your cash in a big vault. They went out of business when they realized they had to pay for the vault but couldn't use any of the money in it. This is like showing ads on something you already pay a monthly subscription for. Remember when cable TV was supposed to be ad-free? That died when people realized they could make more money by double-dipping. And everyone was apparently fine with it.