why is there dark fiber?
Dark fiber (unused fiberoptic cables) is a confusing term for me. Why would any company build so much excess capacity?

The marginal cost of adding one more fiber when digging up a street is very, very small relative to the cost of digging up the street in the first place.  So when you decide to dig a trench across the country, string a fiber across the ocean, etc you typically put more than one in place so that in the future you don't have to do it all over again when you need more capacity.
In addition, much of the cost of the fiber link is actually the equipment at the ends of links (WDMs, receivers, pump lasers for amplifiers, etc).  Burying a lot of dark fiber therefore allows you the option of paying to install more lines sometime later in the future (by buying more receivers) for a relatively small up front cost (just whatever it costs to put one more fiber in the trench).