Why does MacKenzie McHale retain her cut-glass English accent?

Emily Mortimer explains the origins of the accent in this interview with the Telegraph:

While she was forced to make an unexpected temporary move to LA, Mortimer did manage to retain her own accent for the role. “Mackenzie was originally supposed to be American, and thank God she isn’t any more. I don’t know how I would have kept up that accent. Aaron’s incredible dialogue is hard enough in my own accent,” she says. “Very early on in the rehearsals, I asked: ‘Is there any reason she can’t be British?’

"And then Aaron got into the idea of this demented half-breed American-born, British-raised person talking in this passionate, proud way about America. It takes the curse off some of the more idealistic stuff, to have this character who talks in that way without it being jingoistic, without the chest-beating. But it happened mainly because I didn’t think I could keep up the accent."

In my opinion: It depends on the person. From my experience, I am pretty sure of it.

I had a British girl friend who was born in the UK, but lived and worked in the US for at least 10 years (all the while interacting with Americans and our accents). As far as I could tell, her accent was exactly her British accent -- it had nothing to do with the California accent we were living in.

I, on the other hand, lived in various parts of the US over the years -- and my accent subtly changed to some hybrid without me thinking about it or trying to do anything about it.

Actors are experts in creating accents, but they do not have to for the sake of being realistic.