Well, of course, because all dubs are the same!
On a serious note, I actually tend to agree with you: there's a lot of dubbed anime that is just marginal. But it's nowhere near as bad as it once was..."marginal" is a far cry from the word "embarrassing", which is what I'd use for a large part of the history of American localization efforts. There's a lot of decent ones, and not a few pretty good ones. Granted, I'm prejudiced, but so's everybody else. A lot of it simply depends on which one was heard first.
That said, I would disagree that such programs as Cowboy Bebop (probably the most commonly cited example), FLCL, Hellsing, Black Lagoon, and the 2 Ghost in the Shell TV series have English dubs that are in any way (accuracy, tone, flow, emotion, etc.) inferior to the Japanese versions. Having watched both versions of each series fairly extensively (some with differently-sourced sub scripts), I feel qualified to say that it's purely a matter of preference, and mine has already been made clear.