The successful cab owner knows this:
Every ride is custom
People choose a cab precisely because they can ride alone, on their own terms
Empty trips are part of the job, and it's okay, because the next ride will pay for it.
On the other hand, the person who chooses to run a cruise line knows:
Every cruise is designed by me, and people sign up precisely because I chose well
People choose a cruise ship to be with other people, to benefit from economies of scale and to be part of something
Empty trips (or worse, half-empty trips) can put the line out of business
It's pretty easy to get into the cab business. Do a few rides for friends, then list online, or join Lyft, then go full-time.
On the other hand, it's much more difficult to get into the cruise business. There's a critical mass, and the minimum number is a lot more than one customer.
Each business can be a good one if you do it at the appropriate scale.
The warning, and the purpose of the metaphor, is to realize that it's not a matter of gradually going from one to the other. Remember that running a taxi is a fine sort of business, but don't expect to turn it into a cruise ship. And vice versa.