Yoga mats are thin because the flat surface under your feet and hands allows for easier balance when you get into poses like tree or airplane. If you’re not convinced, try standing on one foot on some hardwood floor, and then do the same on high-pile carpet. It’s harder to balance when the thing underneath is soft. Moreover, you can start to really get the benefits of yoga when you begin to understand feedback from the ground and from the rest of your body. The way your body feels on the floor, for example, can reveal that your left foot is flatter than your right.