The large scale members usually mean there is less labor cost and mobilization time, and hence overhead (insurance, fencing, trailer, etc.). Like I said pre-engineered metal buildings are about the cheapest cost per square foot you can get at scale.
No commercial solar farms use large members to save on labor costs.
Standard ground mount equipment also uses pre-engineered metal, just of much lighter weight due to supporting a small fraction of the load.
Incidentally, they are not “I beams” and technically I-Beams are Wide Flange Shapes and used vertically as well as horizontally, and designated with as W(n)x(m) where W means wide flange, n is the depth in inches, and m is the weight in pounds per lineal foot. W shapes are rolled in rolling mills.
Bents are fabricated from steel plates to a design specific to the structure.