Sizing the hash map appropriately is one of the tuning knobs to usually achieve the O(1) desired complexity, in theory you are right, but tuning away from that

Sizing the hash map appropriately is one of the tuning knobs to usually achieve the O(1) desired complexity, in theory you are right, but tuning away from that in practice this does not or rare enough happens is the goal why you actually chose a hash map for hopefully close to O(1)?