I suspect that part of the problem is that some people are expecting any SAN loss to be noticable to the loser, or be a major, mind rending incident.
That ain't so. Much of our IRL SAN losses are not noticable, they happen over long periods of time, changing our world view, our morals, etc, etc. So likewise in CoC. A SAN loss of a single point (or even a few points) is not necessarily a noticable event. But it adds up, and we change along with it.
See enough zombies and otherwise inconceivable alien-ness, and your whole world view becomes....insane. YOU change. Yes, you "get used to it", but that's because you are not as sane as you once were.
Perhaps we also need to define "Sane". I would have to define it in direct context to the 'Real World' and what we expect, and are expected to be, in this world. The Cthulhu stuff changes those expectations, makes us see that the 'real world' that we hold dear isn't quite what it's cracked up to be. We become less 'sane', because we come to know that things are really NOT the way we are taught to expect...
In any event, I would strongly recommend reading "The Illuminatus Trilogy". I have owned five copies of that book over the last 20 years, having lost three (four, but one came back years later) of those copies over time. I have to say that NO ONE I have loaned that book to has been unchanged by the experience. (Interestingly, it's not the story that does it, or even the specific content. It's the overall content, or gist of it all that does it.)