Because hiding for rez was the best way to play Mercy, at one point in the game.
But that point hasn't been for a fucking loooooooooong time and people haven't relearned how to play her.
Hiding for rez is some Season 1 shit. We're on Season 4. People need to wake up.
When I read your thoughts on Mercy, you've been 100% on the money. I haven't consistently played Mercy since season 1, one reason was everyone told me I played her wrong (and Lucio has since always been meta...). And at the time, I think I would agree, my play style did not mesh well with how Mercy was being played for a long time. The last buff changed my life, it allows me to be more aggressive with my rez.
Last night, there was quite a few times that I popped a rez to keep the momentum going, I think one time I solo-rezed a tank (maybe Owzers?) just to keep numbers and front line intact. Last night was the first time, no one said a (negative) thing about my rez's, it was a very surprising to me because I get in arguments with randoms whose thinking that it should only ever be used on 4+ (3+ if OT).
Being able to stay alive once you burn a rez, then being able start building it up shortly after has been a game changer (instead of dying due to a kamikaze rez). Being able to tell someone, "I'm already at 55%" when I burned an Ult not even 30 seconds ago tends to shut people up pretty quickly.
Btw: You were right so far, the Mercy win streak is real!