harry potter - Why are people surprised that Hogwarts is non-apparateable?

I don't think everyone is surprised. The only people who mention Apparating into, or Disapparating out of, Hogwarts are students (generally Harry and Ron, repeatedly). Why would students be surprised/not know that you can't Apparate into Hogwarts?

They don't read Hogwarts: A History

The only place that particular piece of information is ever mentioned as being recorded is in the book Hogwarts: A History. Up until Harry's second year Hermione seems to be the only student who has read the book and remembered parts of it. Even in Harry's second year people likely would have only read the parts concerning the Chamber of Secrets (since that was why they all checked it out of the library).

They're unfamiliar with Apparating

For Muggle-borns this is obvious: they're unfamiliar with Apparating because they're unfamiliar with almost everything relating to the Wizarding world. For those born to at least one magical parent, they may not actually know anybody who does Apparate. We know that it's dangerous, that you have to pass a test in order to do it, and that a lot of witches and wizards don't bother with Apparition, preferring slower but less hazardous forms of transport.

As a result, many students are likely only familiar with Apparating on a very, very basic level. They know what it allows you to do but have no idea how it works.

Different expectations between a public place (a school) and a private dwelling

At least parts of the Ministry of Magic can be Apparated to. The Ministry of Magic is, essentially, a public place, since it accepts visitors and not just government employees.

If you can Apparate to the Ministry, why wouldn't you be able to Apparate to Hogwarts?

They've just never thought about it

If your home has been magically protected against Apparating, unless you're explicitly told that's the case you're probably never going to think that it is. Even if you notice that nobody ever Apparates into your house, you'd probably just think that wizards Apparate to a spot outside your house because that's good manners, not because they're physically unable to do anything else. I think it's unlikely anybody would ever notice that nobody Apparates into their house, though.