The main business of banking is actually leveraging the capital of their owners (shareholders) to lend. Deposits are not the main game, and are there for two reasons - firstly that lending produces deposits, so banks may as well be able to hold deposits just for that reason, but also because deposit inflows create the liquidity banks need to lever up their capital. This is the real reason why banks pay interest on deposits - to encourage people to transfer money in and not transfer as much out. Actually just having the deposits sitting there doesn’t do much for the bank, so the bank more wants you to transfer money in to increase your balance and not just hold it.