[…] why not have a watch sized device that can store insane amounts of power?

Because Hiroshima was leveled by “only” 20 MWh (cost ranges from 120€ in northern Scandinavia to 1010€ in Greece) so having people carry energy wallets with enough to make more around day to day is like paying your groceries bill with C4 (which is perfectly save as long as there is no primary explosive).