you really need to look into fractional reserve banking...

they are literally making money out of thin air.

It works like this:

You put $100 in your bank account
Bank is required to keep a 10% reserve- $10
Bank loans out $90 to me, and I deposit it in my account
Bank now loans out an additional $81 (the $90 I deposited - a 10% reserve) to Bob, Bob puts that $81 in his account
Bank now loans out an additional $72.90, which someone else puts in their account
Then $65.61, Then $59.05, etc

This allows them to create debt in the amount of nearly $1000 ($999.96) off your $100 deposit, which they are owed with interest.

If you request your $100 back, they have still made $899.96 + whatever interest they decide to charge off it (up to say, 25% on a credit card)

Since the 'reserve amount' can be covered wholly by their accounts receivable, they never really need to hold actual cash anywhere unless it's requested. This is why you can't necessarily ask for your entire balance, or why there's limits on withdrawals per day. It has ZERO to do with security and everything to do with protecting the banks from everyone deciding to take their money back at once and the bank not being able to meet their reserve requirements.