Manchester City have a manager that knows how to rotate his squad. Secondly, Manchester City have always had a big squad. They know how to give their players rest. Thirdly, since Manchester City have more quality squad depth than Arsenal, they can afford to rest some of their key players in a match and still be able to win that match.
De Bruyne was injured and they still managed. Rodri is injured and they're still good. That's because they have players that can fill the shoes of these players to an extent.
Speaking about Liverpool. They won the Champions League and won the EPL the next year and what has happened to them since? The best they've finished is second in the league. They even went to the Europa last season. See how poor they were due to injuries? What do you think happened after those two high-flying seasons?
The kind of football Liverpool play requires a lot of rotation because they're always pressing and running so players are vulnerable.
Don't underestimate fatigue. Not every player can play 50 matches a season and still play another 50 matches the next season while having a very short break. A break where they might be representing their national teams during that time.
These players train twice almost every day and play twice almost every week so if they're not well rotated they can either get injured or get fatigued.