I presume the Bank-Liverpool Street OSI is due to historic Network South East fares having a Waterloo-Liverpool Street routing that avoided the tube by walking from the BR station at Bank to Liverpool Street. The Marylebone NR-Paddington NR OSI seems to exist for similar reasons.
Because Monument is the opposite end of the station from the Waterloo & City line, presumably that is why Monument-Liverpool Street is not done. However, one should note that the similar Bank-Fenchurch Street OSI works at Monument despite the Circle and District providing journeys to the much-closer Tower Hill where the change to c2c can be made (and much more frequently than to Liverpool St).