Every cruise we have been on (even some of the mini cruises operated on the North Sea and Biscay ferries) has had that sort of trip offered at every port of call by the cruise company to the passengers, sometimes with stops and sometimes without.
The tours organised by the cruise companies are favoured by those who like to looked after every step of the way and know exactly where they are going and what they are getting (including the time and number of toilet stops
), or are elderly or infirm. The more active and independent passengers are likely to look at Dover from the ship and decide to take the bus into town and walk the rest.
In bigger places we like to use a hop-on hop-off tourist bus as it's usually cheaper, but if these are to be successful you need two or three an hour calling at each stop which ups the running costs, and Dover just isn't big enough to support that sort of service.