It could be because the Baggins family had historically been of higher status than the Gamgees - given Sam was employed as Bilbo and Frodo's gardener, it's likely his family would have been from a poorer background of farmers and labourers, and thus he would have developed his farmer-type accent from associating with other poorer families, compared to the (relatively) wealthy Bagginses who would have mixed with other Hobbits who have developed more refined pronunciation. You've repeated yourself my friend!
Unless John Cena is skilled at camouflage (which is unlikely given his history as a wrestler), I can't see him