Verizon and Sprint, and to a limited extent some of the other cell site operator/owners are in a unique position to lay all of that fiber. Without boring you silly with the details, portions of that fiber are second-layer lay ins, which means, basically, this:
The first time they built the cell towers, they installed enough fiber to carry the signals they needed and only a bit extra, for redundancy, emergencies, and customers fat with cash who wanted a tube or two. But the FCC's rules changed several years ago, mandating that any public network that wanted to has the right to demand co-location access, which is a direct tie-in to another company's network. That means, for example, Comcast doesn't have to route all it's traffic to the net, and the net route it to ATT's network (assuming that's the destination). Comcast can ask for and receive a place to install a fiber link and run fiber that will feed traffic to ATT directly by light rather than by wire, significantly reducing net traffic and greatly increasing speed.
All the big companies do this... they all colocate with each other. It wasn't well received at first, but now everyone is used to it and we all benefit from it.
Back to Sprint et al: Because of this colocation thing, companies got the bright idea that colocation could mean hanging equipment on a cell phone tower. Things like microwave transmitters that could beam a signal across a bay for 1/20th the cost of laying fiber either under or around the bay. The cell phone tower operator/owners smelled blood in the water (cash, actually) and immediately embarked on a project to lay fiber outward in a few directions from cell phone towers to major business centers and other potential "interests."
Many of these big-time fiber users do not rely on ONE fiber, some have a "ring" type system that will automatically switch if one leg gets cut, sort of like how ancient network architecture worked. Others use two or more redundant paths. Refer to my above post about reciprocal agreements with fiber and you end up with what you have now. A whole bunch of Sprint's traffic is carried on other people's fiber, and a bunch of other people's optic data is carried over Sprint fiber. Just fill in the names of any two biggies and that's what you have today.
Like anything else, the first guy in something new is always best positioned to expand it, so in Sprint's case, the revenue they generated was smartly reinvested to lay more fiber. Being they've done this over a decade and a half, they now have significant amounts of it in the ground, while others like Southern Light, are gaining. ATT hasn't laid much because they've been focused on fiber for their video/Uverse project, and that has sucked up all the money able for fiber.
Many of the FL cities (especially the ones down your way, JonB) are getting away from just letting everyone and their brother lay cable wherever and are going toward better municipal planning things where this kind of stuff is planned and contracted. It's enormous money... imagine getting the contract to lay all the fiber in South Miami. This is one reason, I'm certain, Google is putting such emphasis on local government cooperation. They won't want to be forced into dealing with exclusive regional carriers.
Oh... also the feds and the FL legislature change their minds on all this about every 11 minutes. So it's not like you can make a long term plan and expect things to be the same a year from now.