So in my latest game I made it to where there is a reasonable possibility that sol could be invaded in a decade or two (sadly it was not) I originally started with 4 nprs and added 4 more about a decade in with a 250 system limit. I have yet to actually met anyone exploring 18 systems so far, but the nprs seem to be having fun fighting since my game keeps getting slowed down to 5 minute turns. I later turned down the max systems to 150. So, why is it that one has yet to enter my territory? I just checked and 92 systems have been generated.
On a side note my npr generation is 25% and I have found 3 class 2 and at least 5 class 3 planets and I have yet to find an npr on one. Why is that? Am I just getting unlucky with dice rolls?
Possibly.
A reason could be that you've put too many NPRs and they're all busy killing each other to actually explore. Another reason could be that you haven't quite explored hard enough, either, as the best way to make a trail of systems that lead back to your home planet is exploring as many systems as possible.
Mind that with NPR's blowing each other up, they'll be stunting each-other's growth a little bit due to all the blowing up going on, so they won't get as widespread.
If you want more of a challenge, though, you can enable the challenge races (Swarm, EG Invaders, and such.)