Google on the other hand is apparently doing home-run fiber from each house to a central location, where it can aggregate the bandwidth into ludicrously fast switches and hand it off to 100Gbps etc backhauls. That means that (with guessed but plausible numbers) instead of e.g. 50 houses sharing each 2Gbps for an average of 40Mbps with Comcast, you would have 1000 houses sharing 100Gbps for 100Mbps average with Google. Yeah, the "peak theoretical" is higher, but the actual effective available bandwidth is very different.
Then there's the fact that with a home-run fiber to each house, Google can easily upgrade their aggregation equipment and backhaul links in order to boost total shared bandwidth, without having to go out in trucks and mess with fibers again. Comcast OTOH would have to go around and split all their GPON loops in half and hope they can get those new sub-loops run back to their agg points. Heck, there's nothing stopping Google from upgrading the transceivers at each end of the fiber for a given house to make use of more advanced optical techniques, because the fiber isn't shared.