Ask hn: Why does the web assume everyone works sitting at a desk?

There seems to be a very common assumption throughout articles and blogs that everyone works sitting down at a desk in a mostly sedentary job.

I don't recall ever reading article that considered maybe the reader doesn't work at a desk. The intent or advice of the article becomes irrelevant for anyone that doesn't work at a desk.

Excuse me if this seems a bit of a rant. I just find it an odd assumption when people doing non desk/sedentary jobs are so common.