Playing StarCraft 2 as a kid helped me develop the general purpose skill of deliberate practice, and taught me that practicing things makes you improve. I believe that SC2 provides an almost uniquely good arena to improve that skill, when compared to other modern games.
In order to deliberately practice a skill, you need feedback about whether you are doing better or worse than you used to be after you make a change. In SC2, you are playing a one-on-one game, in a limited pool of maps, with no inherent randomness in the game. This makes improved play result in immediate feedback – if you used to not scout, and you were getting rushed, you can start scouting, and start winning against those players. Because the game is one-on-one, you have no one else to blame for your losses, and no one else to carry you to victory. If you win, you did it all by yourself. If you lose, same deal. And the lack of inherent randomness means that your losses really were your losses. If you play Hearthstone, you might have a really bad series of card draws that lose you the game, or at least are the proximal cause of your loss. In StarCraft, if you lose, your keystrokes and mouse clicks were the only thing that could have been different.
Contrast this to League of Legends. In League, you are playing five-versus-five. And it’s not five separate one-on-ones, but a team game with the possibility for each player to impact each other player. If you jungler doesn’t support your lane, you might just… lose the laning phase. No issue of your own, but you can’t have come out on top in what would amount to a two-on-one. In that game, better or worse play may impact the outcome of the game, but you’ll always have an exogenous factor to blame. So, you can continue to make mistakes and not realize you are making them, because you always have someone external to blame. Sometimes a lucky crit will land a lot of damage on your opponent early on. If that happens, and you win your lane matchup, how much credit can you take? Not that much, but that means that you can’t see the direct impact of your skill in the game.
Fortnite and other battle royales are the worst offender. One hundred players, in a free for all, randomly scattered throughout a map? You can play optimally and still lose, against a player that dropped in right next to a better set of weapons than you did. Any game where you can play optimally and still lose will be hard to improve at, because if your play is deviating from optimal, you can’t tell whether your losses are due to your mistakes or random chance.