One way I think about this is the saying “you are the average of the 5 people you spend the most time with”. Try not to waste a slot, you only have 5.
Sometimes managers delude themselves into thinking “it’s okay, most people on my team are excellent”. But a few folks who aren’t as good as everyone else is like control rods in a nuclear reactor: they slow down the chain reaction (of ideas, initiatives, execution) by a lot.
Another analogy here is imagine you’re on a soccer team, and you’re looking downfield for a split second to figure out which teammate to pass the ball to. If most teammates are excellent, you’re still doing the work to think through “well, person A is great but not that fast and their closest defender is fast, so maybe I should pass to person b, but he has a slight issue catching a pass on his left side in a dead sprint (and I’m on his left side), so maybe person c?” Meanwhile, in an identical situation where every player on your team is excellent you no longer think about each person’s relative skills you literally just look at the field and pass to the person in the best position. That’s it. The mental calculus you have to do is way simpler.
Having even one person that’s not excellent on the team means everyone now has to think a lot more, face more friction, with every pass.