Because it's super easy to do with the mask effect... ...or SliceX
Do you mean making the mask effect (or SliceX) on the duplicated clip over the primary clip?
It's how we have to do it till now, but it's absolutely not the elegant way... to have a duplicated connected clips all over the timeline, applying a mask on them, then compounding all that mess... what a wasting of actions : )
Or maybe I just got it wrong?
In Adobe Premiere the masking option is right there in the keying effect just in one button (but I don't like PrPro)
PS: speaking of SliceX...
I've got a Coremelt's Everything Bundle on this black friday - a must-have solution!