I was thinking that they would probably overbalance if they did but thinking about it they can sit up on their back legs. I would think it's something to do with the way their front legs bend. They bend forwards so that the couldn't reach most of their bodies. Having said that we did have a dog who used her front paws to scratch at her face occasionally which is, I suppose, one of the few places she could reach. I would be interested to know what the "scientific" reason is.