Why shouldn't I write my own tools to generate configs?

You could create the tools to do that yourself, and historically many folks have because there wasn't any alternative, but why would you now that Netomata's tools are available? Are you going to write your own web server, instead of using Apache? Code up your own monitoring system, instead of using MRTG or Nagios or whatever? If good tools are available, it's smart to use them to leverage your efforts and expertise.