> Or better yet: why aren't they offering an unlimited data plan like what T-Mobile and Sprint (Google Fi's "partners") already offer?
Because they're acting as an MVNO, and have to pay Sprint and T-Mobile for each MB they deliver to the handset. Google is not trying to replace carriers, they're trying to improve the experience as an MVNO (they providing what Republic Wireless does on a much larger scale).
They ride on Sprint, who is desperate for revenue. Also, Republic retains the right to boot you if you use too much "unlimited" data on Sprint's network.
> They ride on Sprint, who is desperate for revenue.
Google Fi also rides on Sprint (and T-Mobile, though it doesn't seem like T-Mobile is nearly as desperate for revenue at this point). So there's not really a difference there.
> Also, Republic retains the right to boot you if you use too much "unlimited" data on Sprint's network.
Pretty much all carriers have that bit of legalese in their respective agreements (even T-Mobile does, IIRC, and I know for sure Sprint does). Usually that's reserved for extreme cases where one's use of the network significantly dwarfs that of the rest of the user base and is actually causing congestion issues; a few extra gigs is a drop in the pond in comparison.
Republic Wireless also has a roaming agreement with Verizon, for areas where Sprint's network is lacking. But they are really touchy about abusing that. I assume it's very expensive for them.
Because they're acting as an MVNO, and have to pay Sprint and T-Mobile for each MB they deliver to the handset. Google is not trying to replace carriers, they're trying to improve the experience as an MVNO (they providing what Republic Wireless does on a much larger scale).