I also grew up around an interesting mixture of Southern and Midwestern accents and wound up with what people receive as "no accent". It may not be as "generic American" as you think, though. I've found in my own case that my own accent is just naturally incredibly accommodative (also the fun technical word for it) and I reflect back the accent of the person I'm talking to more and more the longer I talk to them. It's generally not a conscious thing and it's weird to try to catch it consciously or consciously trigger it. When I do try to consciously watch it, it is very subtle (it's not trying to mock or parody the other person, as you might if you were trying to consciously "put on" an accent).
(Also, I don't think it is necessarily restricted to cities with mixtures of accents. TV has done a lot to build both a rather homogenized "generic Middle American accent", but also I think has been one of the forces for creating the very accommodative accents from hearing what diversity there is between local/regional accents and the TV generics and how those influence each other, particularly because TV hasn't been intentionally homogenizing things in a specific direction.)
> It may not be as "generic American" as you think, though.
It definitely is. I've had people from a variety of US states tell me I have "no accent" (which roughly means, "American standard West Coast television actor accent").
> I've found in my own case that my own accent is just naturally incredibly accommodative (also the fun technical word for it) and I reflect back the accent of the person I'm talking to more and more the longer I talk to them.
My brother is like that. His speech patterns are chameleonic and follow whoever he's talking to. He does it totally unconsciously and naturally, and it seems to work well for him.
I'm too self conscious to do that to the degree that he does. I would notice myself doing it and it would feel inauthentic (even if it wasn't). I have a fairly consistent accent regardless of who I'm talking to.
(Also, I don't think it is necessarily restricted to cities with mixtures of accents. TV has done a lot to build both a rather homogenized "generic Middle American accent", but also I think has been one of the forces for creating the very accommodative accents from hearing what diversity there is between local/regional accents and the TV generics and how those influence each other, particularly because TV hasn't been intentionally homogenizing things in a specific direction.)