They still need to borrow reserves due to different regulations. For example, Basel III requires all banks to have sufficient liquidity. An excess reserve of bank counts towards bank liquidity requirement, and with reserve requirement being zero every reserve is excess reserve. Also excess reserves have very high liquidity so they are more attractive compared to many alternatives. Consequently, bank still need reserves to operate there is just no longer need some pre-specified number of reserves, the actual amount depends on what the bank does and their balance sheet.

They still need to borrow reserves due to different regulations. For example, Basel III requires all banks to have sufficient liquidity. An excess reserve of bank counts towards the bank liquidity requirement, and with reserve requirement being zero every reserve is excess reserve. Also excess reserves have very high liquidity so they are more attractive compared to many alternatives. Consequently, banks still need reserves to operate; there is just no longer a need for some pre-specified number of reserves - the actual amount depends on what the bank does and their balance sheet.