Per https://unix.stackexchange.com/a/10816/6388, referencing https://www.pathname.com/fhs/pub/fhs-2.3.html#PURPOSE18, "Source code may be place placed in this subdirectory, only for reference purposes"

One presumes calling it "src" is in historical alignment with UNIX using short names wherever possible to reduce typing (and the possibility for typos)

See also: "ls" (list files), "mv" (move), "cp" (copy), "/usr" (userland-specific items), "/bin" (binaries), and on and on