Too long, didn't read. Garen is virtually infallible and thus cannot receive a large power budget. Now, onto the topic.
Garen cannot fail, and this is a problem! :O! Let's talk a moment about failing. You know what makes Jhin's ult fun for both teams, or at least, fun for him when it is hitting it, fun for the enemies when it is hitting the 9000HP Garen in front line. Jhin's ult can fail! Ah yes, you can fail to hit Jhin's ultimate, or better yet, you can make Jhin fail to hit his ultimate, you can body block it, ~~hide behind your low HP AD carry~~, or dodge it, forcing him to fail. Isn't failure great?
You know what else is great about failure? When failure is possible, success matters. If something is impossible to fail at, the reward to the person succeeding is meaningless. Victory is not triumphant if defeat was never plausible. This is why failure is great, because it makes success great.
Back to Garen, Garen cannot fail. At least, not to a reasonable extent. Obviously there are the obvious things like the player clicking the wrong target with his ult, or Garen activating his Q 1000 units away from a trap surrounded Caitlyn. But in general, there is no opportunity for Garen's abilities or kit to fail. You click on somebody with his Q, it does damage and silences, unless they can hop away before it goes off(and it probably will anyway), they cannot make you fail, you cannot fail, and thus succeeding at it is meaningless.
Garen's entire kit is this way. Other than using his abilities at the wrong time, misclicks, or impossible situations where his kit just doesn't provide the tools for him to win(these apply to all champions), Garen does not have sufficient fail conditions in his kit(such as dodging), he cannot fail and his enemies cannot make him fail. As such, his rewards for succeeding are very minimal, both in terms of fun and kit power. Also since his enemies cannot make him fail(again barring some basic things that apply to all champions like shielding his damage or kiting), there is no thrill for them in whether he succeeds or fails.
This is why Garen(like Annie and a few others) see such minimal skill growth and investment worth in players, because if failure was virtually impossible to begin with, then there is no room for improvement other than general strategy and mechanics of League.
Now let's look at Darius. See people talk about 'stat sticks' and 'outplays' and 'counterplay'. I really don't care about these words. What I care about is failure. Darius may be 'bullshit' to you personally, you may view him as a 'stat stick' that has no 'counterplay'. But the fact is, Darius can fail, what is more, you can actively cause Darius to fail. If Darius uses his Q, he can aim it incorrectly, you can dodge it, he can wiff hitting all the targets and only hit some. Darius can fail at hitting his E, and again you can make him fail to hit it by dodging it. Darius can fail at his stack mechanic/ult, you can also use that his damage ramps up to cause him to fail at getting maximum benefit out of it.
As such, when Darius is viable, and when Garen is viable. Darius gives players much more of a feeling like they actively partook in his failure or success. Garen only doesn't feel frustratingly infallible right now because he lacks the numbers and options in his kit to make him meta. Because of this, Darius can get a much higher power budget, in not just damage, but stats, utility and sustain. This is why Garen is not allowed to, and should not be allowed to get as much power as Darius, until the day Garen is reworked and made to have more fail conditions like Darius, mechanics that both he can fail, and his enemies can make him fail.
This question of fallibility can be applied to any champion in the game. So let's stop using convoluted and often loaded words like 'counterplay' and 'agency' and simplify it. "Do the mechanics of this champion leave sufficient room for them to fail, and for their enemies to cause them to fail."
What the potential for Failure Allows:
Higher power budget
Potential for skill growth
More fun when succeeding
More fun for enemies when you fail
Less frustration for enemies when you succeed
Healthier overall gameplay
Thank you for reading, if you have anything to add, argue or reinforce from the post, please feel free to comment.
Sincerely - rovinmain