Per cell site? Got a link for that stat? I'm interested in learning more.
They are deployed depending upon projected traffic per cell site, which is based upon network surveys of the area. So yes, some cell sites have dedicated fiber runs (metro, high traffic - think adjacent to freeways, subways, high foot traffic areas, etc.). Some obviously are not (middle of BFE america and Cellular provider X just wants to say that they have 4G coverage there - so it is microwave linked over distance to other sites, with only a few of those having fiber to the tower).
Here are a few links describing how 5G is going to utilize about 100X more bandwidth than 4G does and why that will necessitate Fiber To The Antenna (FTTA) in many cases, and what FTTA is:
Fiber to The Antenna (FTTA). Testing Tools and Best Practices for FTTA
The FOA Reference For Fiber Optics - Fiber To The Antenna for Wireless
What is Fiber to the Antenna (FTTA) and why does it matter?
Here is an older discussion among some industry professionals where one guy is kind enough to link together some references of years past and % deployment of towers utilizing fiber connections:
https://www.quora.com/Do-most-cell-towers-in-the-US-have-a-fiber-optic-connection
As you can see, there is a lot of variance in reported fiber linking to towers. As I mentioned, you can backhaul over microwave, but long-term that's asking for congestion problems as video streaming and the like are becoming an increasingly larger percentage of network traffic, and in the end you still have to connect that microwave link to fiber to get it anywhere.
And before anyone asks . . . basically fiber is fiber is fiber these days (the cost of digging the trench is the vast majority of the cost). You can run 1G, 10G, 100G on the same fiber (assuming quality deployment). Its all about the optics and transceivers on the ends (exponentially more expensive as you go up, and there are some distance constraints).
BTW, the few people out there that know how to splice long-run fiber together make REALLY good bank. When Joe Schmoe comes in and rips a fiber link apart with his trench digging gear because he didn't check for a fiber run, these people are called out and get paid mega overtime to splice the links back together, no matter the time of day or weather. Probably more than most people wanted to know.