Because it costs about $5 more to put in a valve which lets them work both ways, and people don't really know that this is a possibility, so they don't ask for it.

Most modern ones are. But the old models don't have a reverse cycle built into them. Also, for efficient heating in low temperatures you might want a different gas. Maybe not as relevant in California, but normally you wouldn't be able to use your AC as a heat pump in below 0 celcius.

Nah. They are one direction only. The heating part is literally just an electric furnace in an air conditioner. Heat pumps have them too, but they reverse where is the hot side and where the cold side is. An air conditioner is always hot side outside, cool inside.