Not all fiber optic cable is created equal, and it does make a difference to the network engineering.

Sure, very local networks can be built on a communal basis, but once you're travelling more than a few hundred feet you need to start engineering your network, and that usually means co-ordinating the link loss with the launch power and amplifiers: if the dark fiber provider adds a splice because the fiber got cut, it can affect the network engineering. Or worse, the fiber provider mis-connected your glass and blew up your Very Expensive 1550nm optical receiver. And the location opportunities for amplifiers can also be limited - especially if you're buying dark glass "to a major switching point".

For these (and related) reasons it's usually simpler to buy a lit wavelength (sharing the network engineering headaches with other users of that fiber), or to buy bandwidth capacity.

Dedicated strand fiber networks can be built, and should be encouraged, but it's not a game for the faint of heart.