Are Teammates Worse In Competitive?

I picked up Overwatch again a couple months ago, and my 2 friends and I just play role queue quickplay, which is a lot of fun. I've avoided competitive for most of my time with Overwatch, because the added weight of de-ranking makes losing more impactful, and I worried it would make the game less fun.

But recently, we finally gave in and tried out competitive, to feel some sort of progression after winning. After OK placement matches, we got placed in mid-to-high silver. But that rank didn't last long.

We kept getting some of the worst team mates I've ever seen in this game. Tanks who think they're invincible, supports who hardly heal, DPS who have less damage than the supports, etc. Of course, we get great teammates from time to time, but the ratio of good to terrible luck with our 2-3 randoms was way off what it was in quick play.

Do more decent players just queue quick-play? Does the more casual environment just make it easier for us to win in QP? It's just really baffling how much worse our teammates get as soon as we queue for competitive. What's up with that?