The cynic in me says: "Because they've decided that buffing and debuffing is boring, and they don't want people to do it, because they don't want to do it themselves. They want people to focus on big flashy things, because that forward advertises better."
I honestly can't think of any other explanation. My first thought was: "maybe they have some silly rules about amount of mouse clicks per skill use" but:
1) their UI is very inefficient to begin with, so it can't be a priority
2) offensive skills targetting multiple characters, like Magic Missle or Scorching Ray work normally, so there is clearly double standards.
And there is of course, wider tendency to change tactical utility skills, such as push, into lethal offensive attacks.
