I won't call you an idiot, but assuming that all cases are simply people being sad is a bit simplistic.

Some people are going to be more susceptible to depression, for whatever reasons, and improving someone's surroundings is probably going to prevent or alleviate depression to an extent, but to people who are depressed now it's somewhat pointless advice.

"Patient is chronically lonely" isn't really a diagnosis, even though it should be.

There are plenty of historical records of people having anxiety or depression.

There's almost certainly a link between the prevalence and modern, always-on culture, but to suggest that it alone is the cause is blatantly wrong.