Same reason why all cars nowadays look like cough drops. Aerodynamics.

It's a common feature added to many objects. My laptop, furniture, shampoo bottle, my backpack, my smart phone etc.. etc.

Whenever I see an aerodynamic building I just laugh, because when I throw my ugly smart phone into the trash, at least the aerodynamics makes it a bit faster while it's flying through the air. But buildings? Why? those things don't even move.