If they charge any kind of fees or interest, monthly or otherwise for those accounts, then that’s how they make money on them, even if they don’t actually make money by lending your deposited money back out, or investing it somewhere else (how banks classically make money).
I mean, a bog-standard Wells Fargo checking account charges like $5-10 per month in fees, which is almost certainly more than they can get from investing/re-lending some guy’s $500 balance.