Hmm lets see, just mentioning top laners that kick Garen's ass.
Fiora, Darius, Jax, Quinn, Teemo. All top laners that are fairly common, all kick his ass. But now the fun part, time to tear your post apart.
High base scaling and damage? He has one of the lowest and weakest early games in the game. It is on par with Nasus on levels of weak early. And his passive doesn't even exist early game so he can't just outlast the laner, its not til about 9 or he gets Spirit Visage that his passive can sustain him, so early game he is poked VERY EASILY to death. And his scaling gets decent since he is meant to melt squishies, he can't touch anyone tanky, so literally anyone else that is a bruiser will beat him. Also just an FYI, current Garen is WEAKER than Pre-Juggy Garen til level 16, so pre16 he is WEAKER than he was.
His passive only ramps up at I believe lvl9 since from that point on, only champs can disable it as well as it actually heals a noticable amount, and even then its still worthless unless he has Warmogs and Spirit Visage, 2 full build items that don't help his offensive at all, and that usually takes at least 15-20 minutes to get 2 items. He isn't Mundo, which actually has a good passive even at the start. His is basically crap til he gets Warmogs and Warmogs does all the work anyways.
He has an excute. Ohh no. He is meant to kill squishies and they have to be low hp for it to even kill. By the time it can kill someone, just about any offensive ult does the same thing. Big deal. So what if it hits 1-2k when they have 500 hp, any ult late into the game hits that hard. Crying out loud, Lee even full tank can hit 1k no questions asked, Syndra with 5 balls is even more than true damage Garen ult. Its basically a single target weaker version of Rivens, since at least Rivens is AoE and actually has decent damage even without the excute damage, Ult someone at max hp as Garen and see how pitiful it hurts.
Outdamage an entire team as pure tank. Not even going to waste my time, you're just an idiot. Learn how to click the graph in postgame and see how much damage a champ dealt. Im sure Garen as pure tank isn't nowhere near it. Hell I might even say pure damage Garen would have a hard time getting top damage in a game.
Extremely simple kit. Ohh no thats so bad seeing as he is meant to be a beginner champ, as well as about 5-10 other champs that are also very binary as him. He must be a special case.
Easy build path. Yep, since he is totally not different from any other bruiser in the game, hell even ADCs build some of his items(Steraks I am looking at you)
Lategame Garen is one of the easiest things to deal with, he is insanely easy to kite and kill. He only has the ability to break out of ONE slow which is also his onyl gapcloser, put another slow on him? Hes screwed, he now has roughly 8 seconds of not being able to get to you or escape and 8 seconds is more than enough to downright kill him or make him so low he can't stay in the teamfight. So literally any mage that builds Rylai screws him, Ashe screws him, every tank screws him since they have a ton of CC and a lot that is AoE to keep him in place to get bursted.
Ohh no, Garen is fed, prepare for his OP True damage ult!??!! Yep, since that so called OP true damage ult does slightly more actually useful damage than the normal one. Most ADCs which are his normal target don't build any defenses, so usually only have like 50mr. So you are literally talking roughly a 50 damage noticable difference in damage. If you are at 500 hp and his True damage ult hit you for 1.5k? Guess what, it woulda hit for like 1400 if it was normal, and wanna know whats up? For such a big number, its not a big deal since only 500 of it matters. His ult can hit you for 1million damage for all anyone with a brain cares about, if it only can hit that hard when you are at less than 30% which for ADCs is usually in the 500 range, then it doesn't matter.
I can't believe I took this time to write this all out, but meh, get rekt scrub.