There is no evidence that garlic does anything but make fish want to eat it. It certainly doesn't cure ich. It might... and might is the operative word... raise the ability of the fish to tolerate the negatives of being infected, and thus look like it "cures" the fish.
It would be like living in a patch of poison ivy, but applying calamine lotion to make it not feel as bad. The calamine lotion will make you feel better, and might even cure all outward signs of the poison ivy, but it will not remove the poison ivy plant. You'll just have to get used to it, and live with the occasional itchy scratching, bleeding, and chance for infection... no problem.
Treating ich with garlic is just as irresponsable.