That's why it's ludicrous. The principle is, if a bank has $1 in deposits, it can now lend $10. Put another way, for every $10 the bank lends out, they are required to have $1 in actual customer deposits. The number may not be exact, as it varies, but this is roughly the correct ratio.
Truthfully, customer bank deposits are a nuisance to the banks.That's why they charge so many fees for, among other things, smaller accounts.
Remember, bank deposits are liabilites to the bank. The cash the bank has isn't the bank's cash.
Banks make the money when they lend, and as long as the Fed prints it, which is unconstitutional by the way. Interest charged becomes the profit.
Walk into a bank and try to get $10,000 in cash. Most banks, most of the time, don't have that (there are many reasons for this). They have to make a call and round up the funds. Sometimes, you may have to wait a day. Imagine if everyone decided they wanted their deposits right now...a run on the banks. The money doesn't exist. It simply doesn't exist. FDIC makes some guarantees, but they can't cover what they promised, if asked to.