In the 70s/80s/90s it was very common to get phone calls (on landline phones) at all times of the day from family members or friends.
I remember being on the phone to friends from school for hours in the evening, I don't even recall the subject matter of the conversations but it's just what you did.
Then as more and more people got on the internet, using other communication mediums like IM, IRC email etc became more and more common, and nowadays I'd find it very unusual for someone to call me out of the blue.
That's how new social norms come into the world. Often, they get invented in response to technological change. For example, prior to answering machines being widespread, the norm was to never ignore the phone ringing. After, it became OK to ignore the phone if it was inconvenient to answer because if the call was important, the caller would surely leave a message.
Now there's a push to normalize texting to see if it's a good time before calling, at least if the call isn't urgent. I support this: it's a very minor imposition on the caller's time and reduces interruptions when the callee is busy, as the content of a text can be checked at a glance.
Simply owning a phone is not an invitation to be called.
You and your circle of contacts may love being called and that is fine for you but don't impose this "norm" on others.
You have to understand people originally got phones so they could be contacted which is in direct contradiction to your assertion "It has always been rude". Actually no it was originally desirable. A whole unwritten social etiquette sprung up around use of phones, in fact, because some people could be rude or found certain behavior rude. But people got phones because they wanted that medium.
Certainly when cell phones first sprung into existence some of those social contracts changed due to the cost and the fact in the US the recipient paid for the call and there were no unlimited calling plans. It could be considered rude keep someone on the phone then they're paying anywhere from 25 cents to one dollar per minute to speak to you.
When texting first came into play each text could be 10 or 20 cents each. Again, sending excessive texts would be considered rude.
Social norms have changed over time and I really don't know anyone young or old who shares your view that they consider any phone call an intrusion no matter the reason.
it's only since texting became a thing that it started to seem more rude to call but like others have said we just ask "ok to call?"
Don't make me deal with a content-free exchange to get to the one with content.
Because it's rude. It has always been rude, but now with the phone in your pocket it's that much worse. She doesn't know what her friend isn't doing at the moment, and shouldn't impose.
The featured article gives a better impression of this than I can... but I'll add that the tragedy of the commons is a factor.
You can't just show up from outer space and scream out of someone's pocket whenever you want. This will hopefully be a fleeting technological phenomenon.