It is written in the Megalodron mission set pdf available on kickstarter Link to the booklets: https://drive.google.com/open?id=1T...user=belenmoreno@corvusbelli.com&usp=drive_fs
I think that has more to do with: The conflict hasn’t officially spread past Paradiso yet. Putting the scenario elsewhere would be greeted with “What are our human forces doing on this alien world?” The El sending left over Megalodrons in a few ships is at least plausible. They’re giant walkers, still in use because they look cool, not because they’re reasonable. :)