Oh, it's nothing to do with thinking your life is important 😂😂
It's just about wanting what is in your own house to be for you and your family, not anyone who happens to drop in. And just because your calendar is 'mundane' doesn't mean that other people shouldn't be able to put more 'interesting' appointments on theirs without people reading them. If anything, the idea that personal comments are for 'the world' to read seems more self important than assuming that they will be private.
From the responses on this thread it seems that most people don't think it's acceptable to read calendars. They are functional items for most people, not objects of display, surely?
I think cards are slightly different. Obviously a MD card is from a child, so isn't likely to be personal, but you never know. They may contain references to private things, or apologies, or all sorts of possibilities. Similarly, people sometimes put messages or jokes in Christmas and birthday cards, that are for the recipients' eyes only. I think looking at them is one thing, but reading them is quite another, but that's just my opinion. I still wouldn't snoop into other people's cards in case it embarrassed them.