If you're a "great" shooter (50-40-90 club, or close to it), it makes statistical sense to shoot 3s.
Let's say Steph took 100 shots, all 3s and hit 35%. That would equal 105 points (100 x 0.35 x 3pts/shot). If he makes 52% of his 2s, that's 104 points.
Basically, the only way shooting mostly 2s makes sense is if your 3pt shot is absolute garbage or if you can make high 50s to 60%+ on 2s.
The lack of defense means it's easier to be efficient from behind the arc. If the defensive rules made scoring more difficult, people would revert back to more of a post/midrange game.
Also, there are dudes that aren't really great shooters period and the difference between their 3pt accuracy and 2pt accuracy is relatively small...might as well chuck it up. Definitely true when you watch college games. Dunk, layup, missed foul shots and 3pt shots.