This is just my observation, but it seems like Standard American English lacks any distinct accent when speaking. Listen to almost any person singing with an accent, and they sound like any American singing! I suppose this could be because America has had so many accents thrown on to it that it's averaged out, perhaps. Is there any explanation for this?
I suspect you're coming in from a false assumption. Accents are a combination of many factors, the most notable being vowel pronunciation but also including stress patterns, intonation, etc. However, in common usage "accent" also means dialect or vernacular which would include slang and colloquialisms, tag questions (as a Canadian, Americans often tell me that they love how my "accent" means I say "eh?" a lot), etc.
Pretty much all of those disappear when singing. They're overwritten by the melody and time signature of the music. Slang and colloquialisms are less prevalent in music (especially pop music which sells to an international audience). As for vowel pronunciation, it's not uncommon to mangle pronunciation in service of the rhyming scheme.
So what I am saying is that one could argue that when singing people have neutral accent allowing listeners to fill in the blanks with the accent they are most familiar with (their own).
For an example of this, listen to pretty much any song by Adele (who is from Britain). Personally, while she's singing I'd agree that she sounds "American" (actually, I'm not American so I wouldn't say that. I'd actually say "she sounds like me") but if you actually listen, her vowels are very British.
I found a source to back up my reasoning but it's hardly a peer reviewed academic paper.
Americans have very long open vowels when speaking so when they sing the don't sound very different.
You guys can argue all you want on this topic but I think it's true.
Some people read this and probably didn't want to believe it because they thought it would be an insult to their accent to sound American, but it's not necessarily sounding American - it's just having long open vowels when you're singing, which you're supposed to do. Americans just happen to speak like that.