Because they're interacting with network stacks using OS-specific interfaces.

  def enable_ip_forwarding():

      os_platform = utils.get_os()

      if os_platform == 'mac':
          cmd = ['/usr/sbin/sysctl', '-w', 'net.inet.ip.forwarding=1']
      elif os_platform == 'linux':
          cmd = ['sysctl', '-w', 'net.ipv4.ip_forward=1']

      assert subprocess.call(cmd) == 0