Because most customers don't care about gigabit. I'm a tech geek and I don't care about it- my existing speed (around 10mpbs, I think) has rarely posed problems for me.
In any case, $120 in Kansas City is not ever going to translate to $120 in New York City.
This is incorrect. It is why places like T-Mobile run commercials about caps and why there are tons of articles and blogs and Twitter comments about data caps. A lot of people care.
That's all mobile, though (and also wasn't what I was commenting about). My provider (Time Warner Cable) doesn't have any data caps- I know others do, of course.