It's just a problem in blind pick I'm sure. Because if you don't know you're fighting Garen, and you send someone foolish to face him, it's pretty likely you're going to get manhandled. Most people don't realize Garen has the ability to carry pretty hard too. So when you feed him 8 kills top, and he finishes getting that tower, that's it for you. The group fights aren't going to go well for you from there. He can drop two people easily in one rotation of skills with just Black Cleaver + Sunfire Cape along with his boots, and even more so if you have an avarice blade on top of that.
The best is when people send an Assassin to fight Garen top. Rofl. I had an Akali come at me one game the other day and it was just like...yeah, you're not going to do anything this game. Congrats. Another guy tried to fight me with Thresh, then started flipping out calling me a noob champ after I crushed him 5x, and killed him and Shaco at the same time during a gank. It's like...top lane is not for experimenting with support champions. If you want to win, you need to pick a Tier 1 Champ you know has an answer to everything with proper play and suits your team setup.