The difference between dark fiber and datacenters is that datacenters need a company with enough revenue to maintain the structure. Most data centers are built like big box stores: super cheaply and not meant to last more than a couple of decades while in active use for their intended purpose. You know that Shopko or K-mart that shut down 5 years ago and is still empty? You see how quickly that building has started falling apart with no company large enough to own the space and maintain it? That's what empty datacenters are going to be like.

Fiber on the other hand is just strands of glass with some material covering it to prevent light leaks and provide strength. As long as that glass is unbroken there is zero maintenance required whether the fiber is in use or not. Fiber also has theoretically infinite bandwidth (they're currently working towards releasing 1.2 terrabit transceivers) and usually when fiber is run they run a big fat cable with dozens of not hundreds of strands of fiber so there's dozens if not hundreds of times the amount of bandwidth of a single pair of strands (and lots of spare strands should a few be broken). Sure old fiber won't be made with the tight tolerances that we have for fiber made today, so you might only get a few gigabit out of a single pair, but that's still a ton of basically free bandwidth just sitting there as dark fiber that you can use to get off the ground then start running your own once you have some cashflow