1. If people keep making and buying un-poofing face parts, the hard drive would be crowded, causing it to break down and forget our face parts, or not let anyone in when it totally crashes.
2. What would you do with all the extra clams after you buy all you want and build & decorate your house?!
3. If you buy all your face parts in bulk (meaning many different face parts at once) you would most likely only use half and leave the rest to rot. It's an advantage to have the ability to update your look every few months. (It gives you enough time to build up your clams.)

Whyville is built like a city, every part of it, including the economy. Clams are like money and in real life, clothes, hair, food, all of it gets grown out or used up. You rarely are able to keep something forever.