Here's why:
* Anglo-Saxon is an outdated term: It's not commonly used in modern linguistics. The language we call "Old English" is more accurately described as a Germanic language.
* American English is a distinct variety of English: It has its own vocabulary, pronunciation, and grammar, but it is not a separate language.
* Dialects are regional variations: American English has many dialects (e.g., Southern, Northeastern, Midwestern), but "merican" isn't considered a dialect.
The term "merican" is often used playfully or satirically, but it's not a serious linguistic term.