They absolutely _can_ lend out more than they take in. They tend not to because their owners are risk adverse and depositors get precedence in receivership. But you can find banks with 6 or 7 to 1 LTD (I’d not want to be invested in one).

You bother with deposits for a few reasons a) banks get a lot of power assuming they’ll play a public good in the form of managing deposits and b) they can earn more using the deposits than they have to pay out to depositors.