Learn English – Where does the General American accent originate

You might find this article in Wikipedia to be elucidating, though I don't like the term itself, as "Mid-Atlantic English" to me signals the accents spoken between New Jersey and Washington, D.C.

Anyway, it's true that there's a now-largely-disappeared dialect that was spoken both by the upper classes of the Eastern Seabord and by Hollywood actors and actresses wanting to appear upper class.

As a native speaker of Midwestern American English, I don't hear my accent as an "accent", naturally, but I know it's there. Any English speaker will recognize that I'm American as soon as I open my mouth and start talking English (I occasionally do better in other languages), and they'll probably recognize my accent as "Midwestern", if they've ever heard of it.

So it's not true that Midwesterners don't have accents; we do. As Schrödinger's Cat points out, everyone has an accent.

Possibly -- and here's the germ of truth in this myth -- it may be the fact that Midwestern English is the standard dialect for national broadcasting in the United States that people are referring to. Just as RP is standard on the BBC (with special exceptions for Northern dialects), Midwestern is standard in the US (with exceptions, mostly for Southern dialects).

That's all.

Don't believe everything you hear about English. In fact, generally it's a good idea not to believe anything you hear about English; there's an awful lot of nonsense around.