Or because you spend more and there is a lot of you on the internet and also in the real world! -- Yeah you probably earn more as well.
According to me American do more online shopping rather than other countries. If they click on any ads and then got useful online place that they were searching then there is more chance to buy or take service from that site. Therefore visitors of USA and UK are considered real customers while visitors of other countries may be click on google ads but there is less chances of purchasing something from clicked sites.
Online shopping is very common in western European countries & if they are clicking on an ad then they are most likely to purchase that product/services that's why you get more C.P.C from US (just a thought)
this is because most advertisers are from USA and also maybe because US people have more more purchasing power than other countries