I summed up that many times. In higher difficulties, Tracer is more a problem than a real aid for her team, even with her insane mobility, because bots can shoot her easily. Also, even with Mass Rez, it was extremely difficult. So, just floating around in middle of nowhere while doing nothing relevant, surrounded by bots and having to spend 2 seconds in middle of heavy enemy fire, chances are if Reinhardt dies (and he will at some moment) the game will be over immediately.
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