Your body is to soft for that to really work. When you attach it to a chair you impart a lot of energy to the chair, so it shakes. In turn that energy is transferred effectively through you. If you strapped it to yourself you would only really shake whatever part was directly in contact with it.
Think of it like poking the plate a glob of jello is on vs poking the jello itself. Same amount of energy applied to the plate and the while thing wiggles, if you just poke the jello and it just kinda depresses under your finger.