Maybe they aren't so gullible; maybe you are for believing they are; maybe it's hard to generalise about a nation (I assume you're referring to the USA rather than North and South America) of 290,000,000 people!
I do not think they are. Few Americans put up with the appalling 'service' we suffer with in this country without complaining. They don't put up with high prices - compare the cost of a car there with here - and when the price of petrol went up they made such a fuss, unlike here where we pay the same for a litre that they pay for a gallon
I'm not sure they necessarily are. However, in the USA people who exploit the gullible are able to justify their actions by reference to the Bill Of Rights, and politicians and evangelists are allowed to advertise on TV. So maybe Americans aren't more gullible, it's just that their gullibility is more readily exploited.