I deposit 1000 X into a bank account. The bank lends 900 of these to somebody else.
This doesn't work, as soon as you need to withdraw 101 BTC to your Bitcoin wallet, and the 900 BTC loaned was already deposited to the Borrower's Bitcoin wallet.
With fiat currency, the fed always has more money to lend the bank, since the federal reserve is allowed to write checks from their $0 account. So if you deposit $1000 US in the bank, your bank lends out your $900 US, and you need to withdraw $250 US.
Your bank can always give you your $101, as long as they follow the fed's rules and have at least 1/10 the deposit for $ loaned out, because they can borrow $1 from the federal reserve at the currently near 0% interest on reserve funds, etc.