Because the material is probably very thin, low quality. If it is ripping at the bottom near your belt buckle, then the belt buckle is obviously rubbing on the fabric. The neck is stressed by pulling it over your head. Walmart, Target, Old Navy, all generally have extremely low quality fabric, sometimes you get what you pay for.
One of my biggest complaints in fashion now is decent quality moderately priced clothing is hard to find. Either items are cheapo poor quality, or very very expensive designer. Back in the day people were more willing to pay more for better quality, but back then America had more patience with keeping goods for longer periods of time. Now Americans generally think of everything as disposable.
Items targeted towards young people can be crappy quality, because they grow out of everything, so they don’t hold onto a garment long anyway.