Capital requirements (usually something like "a minimum of 10% of the bank's balance sheet has to be funded by equity or deferrable bonds") are far more important to a bank than reserve requirements:
- if bank capital is too high, return-on-equity is lower for a given amount of profit. Hence shareholders don't want too-high capital
- if bank capital is too low (from too many non-performing loans) then the bank will need recapitalisation which is expensive and bad for shareholders, and if it can't get that, it will be wiped out (either lose its licence or get bought out/nationalised/gutted)
So banks are incentivised to not create debt that is too-risky -- i.e. where the ratio of potential non-performing loans is too high for the interest rate they lend at, or the interest rate of raising expensive additional capital would get too high...
Reserve requirements are about liquidity (bank runs) rather than about solvency - but if bank capital can be ensured to be positive and the bank remains solvent, bank runs are much less likely in the first place, and liquidity provision and bailouts to avoid bank runs won't incur losses for the rescuing entities.