It turns out to be a lot more complicated than that.
I remember when heat pumps were first rolled out in our area. Like you said, they were just AC units with a reversing valve.
And they didn't work for crap, and at least where I was living they created such a bad impression nobody would consider a heat pump for at least a decade. To design one that functions in either AC or heat is easy. But one that does both is a totally different animal.