> you can still walk into the bank and withdraw the $100 any time
This is true, but if everyone did this at once it would constitute a run on the bank and it would fall under its reserve ratio. Not to mention the amount of money in the banking system is far far more than physical currency in print.
This is part of why some types of accounts (trading accounts etc.) require a few days to clear transactions. The expansion of the money supply is throughout the banking system and not just cash and deposits.