The most common ratio of picture to print (by far) is 4x6. So why don't smartphones default to this aspect ratio?
While you're probably correct that the most common aspect ratio for printing is 3:2, that's not what happens to the vast majority of photos these days. People look at them on screen, and upload them to Instagram, Facebook or whatever is the social media site of the year. Instagram is purely a 1:1 aspect ratio, Facebook will crop photos to 1:1 in a lot of instances (at least for preview), and so on. Printing is a minority sport these days, so I think it's entirely correct that smartphones don't optimize for that use case.
The default aspect ratio for the photos that a smartphone produces is a function of both hardware (the image sensor's native aspect ratio) and software (whether and how much the phone's software decides to crop from this image).
It's difficult to speak in general to the choices of specific phone manufacturers and models, but one factor that designers may take into account when choosing a default aspect ratio for photos is the aspect ratio of the phone's display and whether or not the phone's designers feel it is important to fill the entire screen with data from the camera (and therefore to use a nonstandard aspect ratio, since smartphone displays are rarely 4x6, or 3:2 for 35mm equivalent cameras).
If I had to guess then it's as simple as this:
Smartphone manufacturers want to use a resolution which matches the aspect ratio of the phone. Imagine if the average user took a picture and then had black bars either vertically or horizontally on the side of the screen, it would look about as amazing as those 4:3 aspect photos that they loop on the 16:9 TV at weddings, etc...