I've got a theory about that. As someone from the US, I've tried pronouncing "about" the way people from Ontario do... and it's really hard. It's a sound we just don't make in our accent. Which is why I suspect that so many Americans say that you say "aboot" - not because you literally say "aboot," but because it's the closest approximation many of us can make to your pronunciation.

In other words, it's a skill issue on our part.