Garen does to much damage to be a tank
Garen does extreme amounts of damage for a character who can build completely pure tank. His kit makes him a near unkillable champ, because he can take enormous amounts of damage, and is nearly impossible to chase him down. He is completely devastating to mages in general, and it is a serious problem.
First off Garen has 0 mana as we all know, add on top of that the fact that his passive ability is essentially a free warmog passive, Garens sustain is unreal. Late game a Garen can push non stop since he has absolutely no reason to ever go back since as long as he doesn't take damage from anything for 4 seconds he can fully heal from 0 health without a warmogs within 50 seconds, and with a warmogs it is less then 20 seconds, with a spirit visage it's more like about 13. That's right, Garen can actually go from 0 to full health as fast as his damage resistance cools down.
Now lets talk damage resistance. A good Garen can end up with around 250 armor and 170 mr in a game which is manageable. Except wait he adds 30% for 6 seconds. This basically means regardless of the amount of pen Garen will still sit pretty at around at least 65% damage reduction, which hey a lot of tanks can do that no big deal, good thing he is easy to pin down!
Oh wait no. He isn't. Because he can remove slows, gain 30% increased movement speed, and also be immune to unit collision. So Nasus slow? No problem, Thresh ult? Please... If you can't snare him stun him or pull him he is gone, and with 40% CDR, he is probably still gone since you have to hit him with a large moon to hurt him.
Now lets talk about Garen's 1.5 second silence. There are two silences in the game that are longer that Garens, Malzahars, which is 2 second, and Cho'Gaths which hits 2.5. An important thing to note on both of these champions is they really aren't all that fast and that is the silence duration at max level and the are both skill shots. Well Garens is always 1.5 seconds, at all levels, its a non-skill shot, and with his slow removal and speed boost he is going to get you with it.
Now lets imagine you are a mage who is carrying. Garen comes at you, you slow him, he ignores the slow, charges at you, silences you, and proceeds to spin to win. And while spin to win does some damage because he went full tank you in general are not in that bad of shape. Oh now a random jinx ult hits you! You are at about half life AND A GIANT SWORD STRIKES YOU FROM THE HEAVENS! It's okay though because you have some mr and some armor you should be fine. Oh wait its true damage. A lot of true damage. around 1200 true damage, and Garen has just done 52% of the damage to you with a single ability. Now you may find yourself wondering, did i just get hit by 4 Darius ults? 2 Cho'Gath ults? or maybe 5 ignites? Now you can say "Well Cho'Gath and Darius can both do more with their ults!" Yeah but to do that Darius and Cho'Gath have to build AD and AP respectively. Garen could have 6 boots of speed and it would still do that much damage to you, it's just a shame that there is no way that Garen could not cast that based on mana level, oh no because that would just over complicate it. Oh and by the way, it can have a 48 second cool down.
Overall the real problem with Garen is that he simply does WAY to much damage without having to buy a single damage item, so Garen can sit there, silence you, let his jungle get two hits on you, and if you aren't a bruiser, and even then, he will probably hit you with an ult and you will die, while being so tanky that you need 3 people to kill him. He is the epitome of a face roll, and honestly I think he is 10 times the troll than Teemo could ever be.
i think you mixed up the wrong champion bud, at least garen can only use his ult once...
: Highly immobile damage dealers that have trouble getting and sticking to their target, but in return for their lack of range and gapclosers, they are tanky and / or have enough sustain and thus can shrug off hits as they wade into the enemy team to do what they do.
Tanks
: Mostly immobile (minus the dashes a lot of them have) damage sponges and CC dealers. The tanks are often categorized in their ability to either initiate, or to defend, while all have a varying mix of both. They deal no significant amount of damage.