Why is Cannabis Sativa considered a distinct form of Cannabis?
C. sativa was originally named by Linnaeus in 1753, long before the plant was commonly used for recreation (in Europe, at least), so the name probably reflects the fact that that species was cultivated for fiber (hemp), just as many plants have the Latin name Officinalis or officinale, meaning that they were used in medicine.  Later botanists considered other varieties to be sufficiently different to be different species or subspecies. (Including some interesting reasons based in the laws of the 1970s: https://en.wikipedia.org/wiki/Cannabis#Taxonomy )

It's also not true that all Cannabis is cultivated.  It can be found growing wild, either native or as a introduced plant, in many parts of the world.