Because blueprints are slow. If I remember correctly they are 10 times slower than normal C+
Like I said you can do stuff with blueprints. You can combine blueprints with C+. But doing stuff like inventory, random level generation or any other complex task should be done in C+. And you simply can't make a full game without touching C+ - I'm not talking about flapy bird or similar kind of games.
That makes sense. It is still a great solution for newbies and for hobbyists vs unity+Playmaker+middleware-$$$-$$$-$$$$-$$$.
With Unreal Tournament 2003 and 2004 Editor I learned how to make shaders , Now I use the same knowledge to build them in Unity with shader forge. I use a lot of the UT 2004 concepts when I am making my AI and when designing levels. I never really liked the editor though.