Hey there!
Banks aren't in the business of being nice and actually get most of their revenue from their relentless overdraft fees. While it may seem like they're doing something nice or encouraging you to save your money, they're really just encouraging you to become a customer. They don't invest your money, since it needs to be present for you if you want to withdraw it.
Interest on your savings is simply a method for getting new customers.
Hope this helped you out! :-)