How many people actually have internet faster than 1Gbps? Transferring large files around on LAN is not a particularly common use case either. The small number of people that actually need > 1gb ethernet can always add an expansion card.
It's actually a bit of a chicken and egg problem. ISPs can't offer it as there is no real way to push it to the customers from the router to the end user devices. Virgin Media in the UK's 1gigabit package is actually 1.2gbit after docsis overheads (they always overprovision it slightly for various reasons), but the router only has 1gig ethernet.
