Banks have absolutely no incentive to provide money to everyone that wants to borrow all their future life earnings now, not because of hard limits on their funds but because they want their lending to be repaid at a profit (when it won't be, you get 2008). So the quantity of money at a given interest rate is ultimately set by the demand of creditworthy borrowers at that interest rate, which is certainly not unlimited.
Central banks were created to stop banks with solvent loan portfolios collapsing due to demands on their reserves, by ensuring banks could always borrow the reserves to back up the numbers on their spreadsheet. The "reserve requirement" (technically replaced by a capital requirement) isn't something central banks tinker with, and it's not "kinda optional" for them to provide enough reserves for the system's day to day needs. Instead, they influence credit action by adjusting the price of borrowing those reserves, and thus the demand for credit in the wider economy.