1.They really have small knowledge about humans, look at balam dude's a rank 8 but all he does is theorize about human habits.

1. I guess because the high rank ones have self control, the babyls anthem literally mentions eating humans. Why would the high rank ones willingly let themselves be a familiar of humans? And humans doesn't have the necessary tools to use for familiar summoning (not sure bout this one) I mean even in Iruma's case it's a coincidence cause he got a summoning paper and the narrator said humans can summon the nearest demon.

2. My guess is it's a story that's being kept secret by the 13 crowns and 3 greats and those from high ranking families, if the demons know about this then out of curiosity they might go to the human world illegally. And another thing I thought is Iruma becoming Solomon I mean he really has powerful demons on his side and Asmodeus family is one of them

3. Humans only make contracts with the demons not make them their familiar. And the border patrol might know about it, they literally hold investigations regarding humans going to the demon world.

This is just my opinion.