Why is the reward for playing a non-burst mage well so underwhelming?

There's very little reason to play a mage over an assassin, because the assassin is significantly easier to play and more rewarding, while being tolerant of mistakes.

Playing Vel'koz against something like LeBlanc is insane. If I make a single mistake or fail to avoid even a single skill shot, I'm guaranteed dead, period; there is nothing I can do to survive once the chains land.

Meanwhile, even if I land something on her, there is absolutely no way for me to kill her unless she misplays horribly. I can predict her and knock her up out of Distortion with my E and unload my combo, and she can complete avoid it by just tapping Distortion again. Even if I land a Q and W, she can once again just Distortion away. If I were the one making these mistakes, I would be dead, every single time.

This same issue applies to basically every assassin. They have too many 'get out of jail free' cards to avoid skill shots, and too much room for error compared to traditional mages.

It's extremely frustrating to play against this bullshit and watching your opponent make mistakes over and over, but not being able to capitalize on them, while still having to play perfectly just to survive.