There are many Americans who are terrified of flying into a foreign city. With a cruise out of Miami, they never have to step on foreign soil.

It can provide many benefits beyond people who are terrified. All sorts of barriers and annoyances can be avoided, such as managing passports, bringing medications, language barriers, differences in legal rights, dealing with customs, etc. This is even more impactful if you're traveling with kids or the elderly.

You still need a passport for most cruises. All the big cruises are non-US flagged with non-US crews and make a stop, if briefly, in a non-US port.

When you finish your journey, you have to go through customs and show your passport. And your passport will usually be checked before you embark as well.

Yes,for most cruises. There are some domestic ones.

US Customs and Border only require a birth certificate and government issued id for US citizens re-entering the US on closed-loop cruises. A closed-loop cruise is defined as a cruise leaving and returning from the same port in the US. This is the vast majority of cruises from the US.

Also many western hemisphere countries including almost all of those in the Carribean have agreements in place which allow US citizens arriving and departing via a cruise ship to need only a birth certificate and government id.

Traveling with a passport is best practice, you for instance are in trouble if miss your ship for some reason, but is not required for the vast majority of cruises from the US.

This is incorrect. Most cruises from the US are closed loop cruises meaning they depart and arrive back at the same port. CBP only requires a birth certificate and government issued id in these circumstances. In addition almost all countries in the western hemisphere which US originating cruises regularly visit allow US citizens arriving and departing via the cruise ship to enter with only a birth certificate and goverment id. In the Carribean the only exception which comes to mind immediately is Martinique. They used to not require a passport but changed this requirement during covid and have not gone back. Although US originating cruises do visit Martinique, I did so on a cruise in February, it is not a common destination.

Every cruise I've been on (US Citizen) required a passport. There are customs getting on the boat and off at the home port.

I have never been on a cruise from the US which required clearing customs individually at a port in the western hemisphere. The ships are usually cleared as a whole by customs upon arrival and most countries that cruise ships visit in the western hemisphere have agreements in place only requiring birth certificates and a government id for US citizens arriving and departing via the ship. Re-entering the US does require clearing customs but for closed-loop cruises from the US, the vast majority of cruises from the US, you can do so using a broth certificate and goverment issued id.

International flights are uncomfortable in several different ways.

In a cruise you can always just go back to the ship, and avoid any random issue.

I would guess that over half of all Americans are within driving distance to a cruise terminal, and that this group is overrepresented in the cruise company clientele.