I think the fiber industry prefers wavelength division multiplexing over simple directional couplers. You can certainly have multiple 10Gbps customers on a single strand of fiber with passive equipment, and many ISPs will happily sell you such a circuit.
(Gpon, mentioned above, is mostly time division multiplexing. I have worked for 2 ISPs that use gpon, and we've never sold anyone an asymmetric plan, except for the "free plan" that Google Fiber had. Gpon is perfectly happy to be symmetric, but it does have a limited bandwidth shared between multiple subscribers.)
The biggest problem I've found is that the Internet isn't really ready for customers that can download at 10Gbps. When I set up a 10Gbps connection for myself, I had a lot of trouble finding anything on the Internet that would send me data that quickly. Even my own servers were AWS instance types that only supported 5Gbps burst. So that is probably the reason why you aren't seeing consumer ISPs selling you 10Gbps circuits... even the servers don't have 10Gbps! A new wave of upgrades are needed for this to become viable; 100Gbps networking equipment is still pretty expensive.
The use case for faster connections isn't usually so that you can download faster from a single source. It's so that you, the spouse, three kids and the dog can each watch their own 4k stream while downloading software updates for six laptops, six phones, your fridge, your microwave and two dozen smart lightbulbs.
But to really get best results, you need that management on both sides of a chokepoint, and you still want every other link in the system to be oversized.
