[bogomipz]

Well, cost for one. If that fiber isn't being used it's a terrible investment. But more generally it's probably not needed. The bandwidth of optical fibre is theoretically unlimited. A single fibre carries many wavelengths, a wave length is like a channel. This is known as DWDM(dense wave division multiplexing.) A single wavelength commonly caries 100 Gbs. Although recently advances have shown 700 Gbs for a single wavelength[1]. The number of wavelengths and hence bandwidth depends on the transmission gear used on both ends of the cable. You can upgrade the bandwidth by upgrading the transmission gear. The state of the art for years has been 8 pair systems but more recently developments have produced 24 and 36 pair systems.

[1] https://www.lightwaveonline.com/network-design/high-speed-ne...