I imagine that they simply didn't worry about their knives much.
Steel knives would probably have just been regarded as a handy tool.... a small part of a much bigger picture.
It seems that when steel gets some rust on it, then any subsequent rusting process is slowed down.
Back in 1976 my family built a wharf on the sea coast. An ordinary 3" steel nail was left partly driven into the side of one of the piles. Dunno why it was there, maybe it was something to do with the temporary bracing. Anyway that nail appeared to stay in very good condition year after year. It was low enough on the pile to be covered by salt water at every tide. It was something of a novelty to my Dad and I who are both tradesmen metalworkers. The nail isn't there now, but that may be because the whole pile has been replaced. The same plain steel ladder is still in place on the wharf though, and we regularly climb up it. It has a hard rust on it, but it still has plenty of 'meat'.
I recall reading a National Geographic article about a wood carver. I think he was from the USA. When he got new knives for carving, he would bury them in the ground and dig them up much later when they were thoroughly rusted and pitted. He seemed to think that this improved the steel, and maybe it did in some way.
It takes a lot of corrosion (which generally relates to a long time) to render a steel knife useless.
There is absolutely nothing wrong with being a knife enthusiast and keeping blades in a shiny condition. But I feel fairly certain that a lot of people who have to use knives regularly in their day to day existence don't have quite the same focus. It is just a tool like the shovel in your garden shed.
I sometimes use a bit of vegetable oil on my blades, but I don't lose sleep if they rust.
