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.

Wiktionary says that there's no variant of the Canadian pronunciation that's actually ‘aboot’.

I can’t speak for others, but out here on the west coast, I say it more like “a-bow-oot”. I could then see how it morphs into the stereotypical “a-boot”.

As a west coaster, I assumed it is, like most of the dumber stereotypes, a Quebec/maritimes thing.

Other than bagged milk, which I know is your fault.