All has to do with throughput and backplane capacity, if you are on old shitty copper infrastructure and your access gear to the customer premise has only a limited throughput backplane capacity, from the ISP standpoint you have to budget that capacity of both upstream and downstream. It makes more sense to put more into the downstream than the upstream because that's what most customers use.
FiOS is modeled around high customer area densities and asymmetrical profiles are ultimately going to be cheaper for Verizon to not have to provide more gear for fewer customers because of high symmetrical profiles even though symmetrical profiles burn up the backplane budget so to speak. Symmetrical profiles are definitely more feasible with fiber access equipment so it is not necessarily wrong to say that fiber is symmetrical but that is not a specific differentiating property of fiber to the home itself, but rather the capacity of the fiber and the back-plane of the gear that drives access to the customer premise.
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