Closet shelf brackets do not have an unlimited weight limit. People are in the stacking habit. If you were to look in some closets you'd probably see the shelves stacked to the ceiling with items and items. Often times this is done without much concern for the weight limit of the closet shelf bracket.
Wire shelves make very sturdy shelving. They can carry loads between 200 and 400 pounds. The weak point in these shelves isn't the shelves themselves, but the closet shelf bracket. Most wire shelves are held in place with a thin metal rod attached to rubber brackets that the shelf sets in. Too much weight on either end can bend these brackets causing the shelf to fail.
Wood Shelf Aluminum Brackets
Wooden shelves need to be mounted in closets with brackets. These are usually L-shaped and screw into both the shelf and the wall. The weight limits of these is a little more than wire shelf brackets, but you still must distribute the weight evenly for the best holding power.
The weakest of all the shelves, glass shelving can handle loads up to about 50 pounds. The closet shelf brackets for glass shelving is usually a sliding system of rails and pins. Keeping smaller items, or very few on a glass shelf will make sure it stays on the wall and not on the floor.
There are alternatives to brackets in shelving, be sure to check them out in our guide here.