>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.
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.