In my experience, some people in the US/Canada struggle to understand a British accent and this might be one reason to dub it into another accent.
My brother tried to order an apple pie fritter in Canada and the woman didn't understand what he was asking for. When he proncounced it fridder, she did :-) I understood both ways perfectly well but hey!
I had this too when I was in the US. A waitress didn't understand "bottle of water" so I had to point to it, to which she replied "Oh you mean a boddle of warder"
That’s unfortunate… variations in American accents can differ as much or more than that, so it’s a little surprising she struggled. I hope you don’t judge all Americans by that experience :)
From my experience, even people used to variation still struggle when it is the wrong kind of variation. It's a bit difficult to explain but I have seen it happening regularly, including once when I tried to say something about Vancouver and it took a writing down to get it across. The other person was used to one southern American accent, Estuary English and European Globish at least.
Not understanding an accent is purely due to lack of exposure - so if that's the reason producers choose to dub everything then it's a self perpetuating cycle of ignorance.
My brother tried to order an apple pie fritter in Canada and the woman didn't understand what he was asking for. When he proncounced it fridder, she did :-) I understood both ways perfectly well but hey!