You are actually complaining that Mercy, who does everything better than every other healer, doesn’t also have an ultimate that makes her better than both/either Zen or Orisa’s.
Dude. Come on. Mercy’s ult falls directly in line with Nano boost and coalescence. Offensive ult with defensive capabilities. It’s not supposed to outclass another off healer. She already does(did?) that to Lucio. And certainly isn’t supposed to outclass a main tanks ult.