Optical fiber cables don’t carry electricity. Why are they covered within the NEC?

   because they are  installing with electrical cables and they are dependent upon the electrical and electronic devices 

For two reasons: first, because they are often installed with electrical cables and by the same installers; second, because they are dependent upon the electrical and electronic equipment to which they are connected.

Optical fiber cable can be deployed in two ways: underground fiber and Sky raping with high tension electrical transmission power. The latter definitely has to be guided by NEC because the fiber is sky rap along with high tension power cable on electrical transmission tower.

Because they are sharing Electrical Cables , also it is installation by the same installer , moreover light signal will be converted to Electrical signal and Vise versa so the should be under the same Standards and regulations

The following are the evidence that they are related with NEC.

1) Powder glass

2) Cover glass for image sensors

3) Glass for diodes

4) Glass for laser diodes

5) Glass for optical devices

6) Glass ferrules and micro capillaries for optical connectors

7) Glass material for aspherical lenses

8) Collimator components

9) Micro prism

You can put optical fibers within the same composite cable for electric light, power and Class1 circuits operating at600V or less where the functions of the optical fibers and the electrical conductors are associated.
Nonconductive optical fiber cables, but not conductive optical fiber cables, can occupy the same cable tray or raceway as conductors for electric light, power and Class1 circuits operating at600V or less

Well as the name suggests, NEC (National Electric Code) covers the standards of electrical wiring and equipments. Historically, control signals associated with the electrical equipments use copper control cables. Referring to the term NEC (national ELECTRIC code), we shall be ok with that!! However, nowadays most of the electrical utilities utilize fiber optic to carry the control signal, example of OPGW carrying SCADA control in between substations. Therefore, NEC covers the standards of fiber optic cabling from this perspective.  

because it's made by a glass material