Competition? Profit motive?

If they go too far one way, people will use other banks. If they go too far the other, they waste money.

Also, there comes a point where they could just invest that money in someone else rather than offer the service themselves, and make more money. That keeps things from going too far that direction.

And finally... Deciding things is hard. When someone else decides things for everyone, legally, it's an easy choice to follow it. Most of the time that's price fixing and is illegal.