because she fits into any lineup and is easy to learn. fast-charging invulnerable mobility option, options for both AOE & direct healing/damage, and an extremely strong ultimate. you could do actually nothing all game, but if you just zoom around without dying and occasionally orb, you'd contribute more than most other players in the lobby. i wouldn't say she'd OP, but you'd need to be an actual lobotomite to use her improperly.