I'd surmise that the main reason leap years continue to break systems is that new software is developed within the time between leap years that fail to account

I'd surmise that the main reason leap years continue to break systems is that new software is developed within the time between leap years that fail to account for them. Date/time handling is notoriously difficult to get right, and leap years are an edge case that's triggered very rarely, so it's easy for novice developers to fail to handle it.