Because going underground is a hell of a lot more expensive. Here in the Washington, DC area 2 projects come to mind. Our "friends" at DOT will not allow the extension of METRO to Dulles Airport to go underground, even though it would be better for the neighborhood, because the cost can't be justified.And our Purple line in Maryland would cost way too much underground, even that's what the local NIMBYs are asking for.
A viaduct could also do the same job and they wouuld have to worry about drainage.
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