Because where is the fun in stomping people with your progosu character, for either party involved?
You're obviously not getting fun hitting a moving target practice duck with your progosu 16 hits 150 % bar combos, and I'm not having fun either. So why?
To put it in HoN terms moody, why would you send Loki as Fayde mid versus me when I said before the game "hey guys, I'd like to try out this new hero I haven't played yet mid"?
Sometimes you're practicing execution with your main. Often, my weird stuff I try in fighters comes from when I've stomped someone, and start experimenting, to see if something silly works. As for the other guy, it's often no fun for them, so if I know I'm outclassing someone, I tend to experiment with chars I want to try myself.
That said, if I don't know someone, I'm reluctant to sandbag, cause all too often I've paid the price and gotten tauntmail, which is irritating. If I do know them, usually they're better then me so I can't sandbag.
Also, sometimes you're so much above the other person, that it doesn't matter what you do, you're going to stomp the person no matter what.
You can't do as much experimenting in SCV, because the movelists are so sparse though.