I think that a large part of the reason is that, not only would they have to spend a lot just to get the cable to the house, they would also need equipment that accepts fiber WAN connection. Short Copper wire works great for high speed, but only if it's in good condition- this would serve to find out whose cable fits that definition.
Spectrum is raising the national minimum speed to 300Mbps, according to the tech support person I spoke with yesterday and even though the customer who needed their modem swapped is now paying for 500Mbps (based on the recommendation of someone at Spectrum, they're not going to see that unless they plug directly into the modem with a computer because he has a death grip on Apple AirPort network devices and Apple ended development of these almost 6 years ago, so they do what they do, which falls short of everyone else's hardware. The last Airport Extreme was only good for 802.11n and that tops out at about 200Mbps, on a good day. The Extreme was good for 802.11ac, but even that's not going to do better than about 300Mbps.
I think a lot of people will be disappointed when they sign up for Gigabit because the rest of their networks need to be upgraded and they won't be very willing to spend what it takes.