Who I have the same questioned this topic

Numerous articles about it if you want to Google it.  This article says that "Samsung decided not to use the Android Marshmallow “adoptable storage” model. We believe that our users want a microSD card to transfer files between their phone and other devices (laptop, tablet, etc), especially the photos and videos they shoot with the camera."

The LG G5 and Galaxy S7 won’t support Android 6.0’s adoptable storage | Ars Technica