Woah that is totally untrue. Brokerages make a lot of money by investing your balance. In particular, IB makes 49% of its revenue this way.

https://www.kalzumeus.com/2019/6/26/how-brokerages-make-mone...

To add an even clearer source to the other replies: https://www.interactivebrokers.com/en/index.php?f=1595

IB is abundantly clear that it makes money from cash balances. I'm not sure where you got this idea from.

Huh? That's just not how this works. There's always a spread between the cash interest that the custodial firm gets on the balances it holds and the interest it pays to the customers.

Interactive Brokers makes 49% of revenues from customer balances.

https://www.kalzumeus.com/2019/6/26/how-brokerages-make-mone...