Hold luggage gets weighed and has a weight limit. The average weight for pax+hand luggage is pretty accurate and gets reviewed periodically as the population gets fatter! Excessive hand luggage will also get weighed.
Smuggling on 10kg extra in your pockets will put extra weight onboard that cannot be accounted for and if this became widespread would probably force the issue of weighing pax and their possessions. Plus, these jackets will causes extra mayhem and annoyance in the cabin.