Lots of bad DPS players in OW.
This is the key to competitive.
If your team has decent DPS players you're already with a headstart. If not then you need very good tanks/healers to play force multiplier, to keep those bad DPS alive. Having one decent DPS isn't enough. In order for a smooth game both DPS need to be good. They're the guys doing the dirty work. If your DPS ain't gold and silver in damage you got a problem.
If everyone is good, then no one is being carried, everyone does their role well and you win. Unless the enemy team is actually better.
If the DPS is bad, then you need good tanks and support to keep them alive.
If the tanks are bad, then you need good DPS and support to keep the enemies from the tanks.
If the supports are bad, then you'll need phenominal DPS and tanks. You're running without any force multipliers, so the skills really need to overwhelm.
It's not so much individuals can carry, but how much the whole team can compensate for bad players.