There are probably tax reasons for offering it and they would prefer not too many people to take them up on it

It is not their core-business like cruises or ferry companies. So there is probably very substantial overhead in coordination, billing and so on. Same goes for anything from any company, technically they can provide service and even might do, but price it is sensible due to extra work is high.

Think about it. A ship needs to include essentially a small apartment (with the weight of a small apartment) for a couple weeks (plus food, heat/cooling, electricity, water/sewer, upkeep for the small apartment) whereas for an airplane you just needs to do that for a chair for a few hours. A cruise ship (yes, I know, includes a bunch of luxury amenities, but still the right order of magnitude) has like 30,000 kg per passenger whereas an airplane weighs about 300 kg per passenger.

So whereas ships are more efficient per ton than airplanes, but the end result is that jet air travel is about 5-10 times as efficient per passenger-mile because it's just a chair instead of an apartment. Plus the capital utilization (renting an aluminum chair for a few hours vs a small steel apartment for a few days) aspect.

I've never done this, but if you read stories by people who have, they appreciate not having 3,000 other tourists on the boat; just a few dozen or so, plus the crew members who don't talk to you.