The main thing, for Bo5s at least, is that both teams come into the series with a strategy for Game 1. One team wins that game. Then, the losing team decides that it wasn't the comp that lost it for them, and the winning team really doesn't want to change anything, because they won. So the teams end up with the same picks in multiple games. It happens much less in Bo1s, since both teams come in with special strategies against that opposing team and don't reuse them against other teams.
And, often, the coaches talk to their team about what went wrong in-game. "God dammit Smurf, stop dying to Odoamne all the goddamn time" might be something that the ANX coach says to Smurf after Game 1/2 vs H2K, for example, or "Stop dying and end the goddamn game" might be something Prolly would say to H2K after Game 1, which they fixed for Game 2.