I think you are thinking of coax (cable) and not FTTH. Fiber has a much wider frequency range and one tends to think in wavelength terms and not frequency. And

I think you are thinking of coax (cable) and not FTTH. Fiber has a much wider frequency range and one tends to think in wavelength terms and not frequency. And rarely is more than two wavelengths used for connecting customers (leaving most capacity of the fiber unused). With FTTH there are two common deployment strategies: dedicated fiber per customer (then there is no reason at all why it wouldn't be symmetric) or (G)PON. With GPON the issue is that multiple customers share the downlink and uplink. And while it's easy to make the downstream burstable (meaning you can use more than 1Gbit/N - with N being the number of customers sharing the upstream GPON port), since only the ISP transmits in that direction for the upstream each customer gets assigned a timeslot to transmit (since GPON only uses a single wavelength for transmit and another one for receive). This means that even if the connection is symmetric at the ISP end (1G down and 1G up) one customer only gets 1G/N uplink bandwidth while they might briefly be able to completely saturate the downstream.