And that is bad game design in those RPGs. It's frustratingly common since D&D, the trendsetter that most other RPGs copy, is also one of the worst games out there about caster dominance in every edition but 4th.
There are numerous reasons why it's bad game design, some of which apply to multiplayer games, some to single-player games, and some to both. In multiplayer games, it skews the spotlight between party members, typically in favor of allowing the wizard to solve most problems that aren't combat-related. In single-player games, it punishes the player for making the wrong choice by picking the good-early class, because the only real challenges in a single-player RPG are at the end of the game. Any challenges earlier than that can be beaten by grinding until the player is overleveled.
The Early Access development model that TiTS follows introduces even more, because again, your "mage" will be waiting years to reach endgame levels. We are at Planet 3 of 10. It's going to be a long-ass time before we get to endgame. Every class should be equally functional at every stage of development.
The only sort of genre where this type of balance works is in games like League of Legends, which are competitive games where matches last about an hour and thus it's a tactical choice the team makes about how much to weight an early advantage versus a late one.