Is astral projection a common thing?
I think it is, since I've done a fair bit of astral projection in my dreams, although I had no waking intention to do so. Best example is dreaming of an actor-friend I knew who got up to disembark from a plane he had just arrived home in, and walked away without his jacket. When I next saw him, I told him about it, and he looked startled and asked when I'd had this dream. He said he had indeed been flying home from a voice-over job in another city, for a documentary film, and he had indeed left his jacket on the plane upon arrival and had re-board to fetch it.
Two observations: I was not involved in the action of the dream (nor in the actual event in his real life), but I was "present" to observe it without being observed; that suggests astral projection to me (something I've never been able to do at will).
And, note the absolute triviality of the event itself. So far as either of us knew, literally nothing has come of this event. This suggests that for people who astral travel in sleep without lucid dreaming or intentions of astrally traveling, incidents of astral travel are usually so mundane - and possibly also so common over their lifetime of sleeping dreams - that they're not remembered by most, let alone spoken of to and validated by someone else (who lived 2,000 miles away, in my case).
I am not so "special" that I can assume that I belong to some select group astral travelers, so I am assuming that this is a talent that all of us have, but that we tend not to use it much or to remember when we do, depending on our background, the cultures we grow up in, religion, etc. In fact, if it's not useful to you to remember astrally traveling (in dreams or otherwise), you probably won't remember, or will dismiss such a memory as fantasy.
So - it's strictly anecdotal, but that's why I do think astral travel is common, just not much talked about (in the West, anyway) or even remembered.