Having an accent gives one an identity. Listen to French or Germans speak English or even Nigerians, Jamaicans and South Africans. In-fact even Tanzanians, have had workshops with Tanzanians and man their English is kinda something else, they struggle a lot.
You have to understand there is a difference between someone who started speaking in English from form one and a dude who has grown in it. My kids here and most Nairobi kids speaks it so flawlessly without a tinge of "mikoani" accent. But you can't say the same for kids from shags, however, that doesn't change the brilliance in them. We have had kids from Maranda high school top the country in languages, both English and Swahili despite the fact they come from rural areas.
In Tanzania though, I noticed there is a breed of ladies from Kilimanjaro who speaks in English so skilfully, you'd think they dropped right from Nairobi, but the same can't be said for the rest of Tz.