The delay before mercy's teammate's can become active is an opportunity for you to rush away or to either reposition yourself.
But that would make i't far too easy to deal with her, not to mention that there is already a time gap before her teammate's can become active.
Mercies dominate the competitive meta because her rez power is too strong. You can wipe an entire team and have your work nullified by a single Mercy, and often multiple ults are used to secure these team wipes, meaning lots of your team is missing their ults, while the enemy team will likely have more.
This power wouldn't be that bad if it took more cunning/skill to do. But as it is right now, the Mercy can just wait around a corner, then fly towards a dead teammate and mash Q.
It doesn't matter if the delay gives you time to run or reposition. I shouldn't have to loving do that because getting the rez in the first place is so god damned braindead.