It isn't mages as a thing, it's mostly that single player games don't need to be balanced. As long as the gameplay experience is engaging and puts players into the flow state when playing, it doesn't actually matter if one class/character/build/etc. is stronger and another is weaker. Players can choose to challenge themselves or take an easier time by using the stronger character/class/build/etc.
It's much more detrimental to give players a major option that isn't particularly engaging. This is can be because the option is so strong the game becomes boring, so weak the game becomes frustrating, so repetitive with minimal things to think about or solve, so complicated that the player has difficulty finding the fun, or any of another thousand problems. Regardless of which reason, the result is the same - the player drops out of the [flow band]. Players that can't get into or stay in the flow band will typically drop the game entirely because the experience isn't fun and there are better things they can do with their time. It doesn't matter how powerful or weak options are relative to each other as long as those options help keep the player in the flow band.
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