The UK fixes this by forcing the companies who own the lines to rent and share them with other providers. ISPs in the US vehemently oppose this, of course.
But is it still required though? Isn't this how Sonic uses AT&T lines? Perhaps it only applies to DSL, which is basically obsolete now?
Internet has yet to be regulated that way and fiber lines aren't considered the same as those copper lines.
Also, fiber to the home is done using most often a PON system purchased from a specific vendor and is a time shared medium. You might only get 1Gbps service but you also aren't getting 100% of the fiber to yourself. The home side device will filter information from your neighbors for instance.
There's always ways to do this of course but, it wouldn't be as straightforward as patch the fiber from your home in the same way that they did with copper lines.
It did make sense at the time of the law: carriers kept clustering around the major metro areas and wouldn't expand to the boonies. Nowadays, that law has outlived its purpose.
The TL;DR is that the cost of cell towers is mostly the same irrespective of bandwidth (e.g., hiring a technician to break-fix as parts fail from weather conditions), so carriers figured out they could "rent" their leftover bandwidth.
Most of the "cheap" carriers are simply slinging together MVNOs from all the carriers (e.g., Boost Mobile, MetroPCS). It creates a weird situation where you get plenty of signal on off-peak hours and (often) the middle of nowhere, but at the expense of having no signal in larger metro areas at peak hours.