Why do people say an "American Accent" is no accent at all?

Ummm... I don't think Americans generally say they don't have an accent. And just because English originated in Britain, doesn't mean it isn't an accent. An "accent" is relative to what you're used to hearing and growing up around. A Brit may not think a fellow Brit from the same part of the country has an accent, but may think a Canadian or an American or an Australian has an accent. An American may not think another American from the same region of the country has an accent, but may think someone from Ireland has an accent. Australians may not consider their vernacular as "having an accent." Objectively speaking, ALL languages have accents. All variations of English are accents, including the British accent. The accent is just more defined when it's an accent other than your own.

In any way you look at it, to begin with, forget the term "American accent" to refer to the way people from continental USA speak English. Citizens of that country should be known as "Unitedstatians" and not Americans. Besides, they have a big variety of accents (ways in which the population speaks English in different regions of their vast territory). As a matter of fact, the same goes for people in the UK. I think there can be no such think as standard English. There are numerous factors that influence languages that they cannot be standardized.

They just don't know (ignorance)

Who says that? To everybody their own accent feels like the "normal" one and some people may think that they have no accent, just because they are used to it. Everybody has an accent. Just travel to a place where you are the one with the funny accent, or ask a foreigner to imitate your accent, and you'll see...

standard english

To An American an American accent does not exist except for the endless regional accents the same applies to the British but although we speak "English" that does not make our particular pronunciation an accent we here say that the British have a British accent and that is as absurd to you as it is to us for you to say we have an American accent.

One can determine regional accents both here in the US and in the UK. The intonation and meanings of words both evolve over time. It is hard to say what dialect would be the standard. I have heard that in the US, the Midwestern dialect is preferred in the media. All I know for sure, is that, I can tell when I am getting some off-shore call center.

The only people who would call a yank accent neutral are yanks. I teach and have taught English in several Asian countries plus know others who have. The Canadian accent is the most desired and considered the most neutral for pronunciation (except for newfies and those from "Turonna"). UK and other British tinged accents (eg. Aussies) are viewed as too strong, and Americans with their odd vowel and consonant pronunciations (plus their VERY bad spelling and grammar) are only wanted if schools want local US information in their curriculums. .

Because it is a misnomer. There isn't an American accent their are southern accents, Yankee accents.... A person with a really good ear can guess what tri-state area you have spent most of your life in.