Duty with all of the excentuated-ness of the “T” would make you sound as if you have and extremely tightened sphincter (if you didn’t already have British accent).
As if my duty was to hold my pinky up while sipping tea when I say duty. Again, only if you were to just decide to say duty in an otherwise American accent.
In some parts of the US kids would probably just get beat up at school for saying it like that. We have to say “doodie” for safety reasons.
Plus it’s just easier to mumble and save our energy for more important stuff 
