They already do, with their prosumer lines. Mommy bloggers, for example, have a surprising amount to say about how to get the most out of a Nikon D3300 or similar; I actually got some useful tips from them back when I was just starting out with my first D5300.

Granted, the camera makers haven't gone to the extent of just building a smartphone into a DSLR so you can post to Instagram with two taps, or whatever. IMO that's because they know where their strengths lie, and also understand that such a hybrid device would need a few generations to evolve into really solid usability - generations which it would not receive, because the same market at which it'd be aimed would consider the v1.0 difficulties a deal breaker for exactly the ease-of-use reasons you're arguing it would be a good idea for the camera makers to do this in the first place.