Usually because they don't realize that as a hardcore Garen player I know the secret to Garen.
You mean keep poke on him to keep his passive down and aa through his spin?
It's not much of a secret.
The biggest thing to shutting down Garen is keeping bush control. If he can't pop out and Q you then you're winning against him. Keeping his passive down isn't 100% possible with a lot of top laners, the biggest threat to someone opposing Garen is his Q.