I've seen too many people with big egos who stagnate as developers. They were the top of their class in school and were used to being the "smartest" in the room. When they start working, they often make a toxic work environment and can be a huge negative impact. Unfortunately their confidence makes them more appealing to the management and many are promoted to positions where they have even less reasons to listen to others "below them" and eventually stop improving

Those that are humble and eager to learn and work with others are a huge boost to the team. The mythical 10x programmer may not exist, but a single individual can certainly make a 10x team.