The "all you can eat" stand in line with a plate or tray and pick what you wish to eat from a table type buffet was unheard of until Herb McDonald came up with the idea in 1946 at the El Ranch Vegas resort on the strip in Las Vegas. It was a simple spread of cold cuts, snacks, and a few hot meals and was served at midnight, as a way to get hungry people who were leaving the shows at other establishments to come and eat. The cost was just $1. It worked, and besides being popular, was cost effective because it eliminated the overhead of keeping an entire restaurant staff on duty. It also provided gamblers for the graveyard shift, a time when casinos are usually the least busiest.
On ships, especially prior to WW2, the term "buffet" does mean a lavish spread of all you can it. However it means you can request whatever you wish off the menu and the waiters will bring it to you. You can eat as much as you like, until the restaurant runs out of your selection. Prior to WW2, the wealthy considered it beneath them to do the plebeian "work" of having to dish up their own food and bring it to the table. They confided that was the job of servants, and their rightful job was to be "served". The idea of an anytime, walk in, grab a plate and load up as much food as you like, then find a place to sit and eat was unheard of in 1912. Too bad nobody had thought of this, it might have been a big attraction.
Cafeterias did exist, in large cities, but again, catered to the working class who punched a time clock. With every minute of a lunch hour counting, going to a cafeteria, grabbing a tray and then having a staff member dish up your food from a prepared selection, then paying for that and eating it was very popular. New York City in 1900 was cafeteria heaven. But it was not all you can eat. You got one portion, and if you wanted "seconds", then you paid for a second portion.
Royal Caribbean in the 1980s was offering a midnight buffet, which was very popular. Now I understand almost no cruise line offers midnight buffets, or not the giant full spreads of foods that they used to offer. At every seating in the restaurant, you could always ask for more, and the waiters would say "are you sure you want more?", or something to try to discourage you, but if you said yes, then off they would go and come back with another plate of this or that. That is how most of the old liners worked. Of the old liners, one of the earliest "stand in line and fill your plate" buffets that I am aware of was Matson Lines in the mid 1950s with the "Aloha Buffet". They weren't being nice to you, they were just trying to cut cost by reducing staff. The advent of tourist class airfares on United and Pan Am with their DC-6Bs was starting to pull away passengers from ships. Now you could be in Honolulu in just 8 hours by air and on the be on the beach the same day.