Sorry for the slow reply but I've been away all day and the thread moves fast. Thanks for pointing me to the info sheet, shame it says so little about the actual life cycle that I think is the missing context here.
Now in response.
First of all I think directed is too strong a word. Avernus has only directed things once in this quest too my knowledge. And that was when it freaked out because servants of the C'tan landed on it. Most of the time it nudges. Frankly I think things like the Mantle Wyrm which was considering sinking a city are fully sapient and generally act under their own initiative.
Secondly even if you are correct that only changes the question, in fact it doesn't even change it that much. Why are Blink Spiders the ones hunting humans? Why aren't we seeing phase tigers migrate randomly around to avoid the void sheilds that are attuned to them? Why do we only have to deal with dinosaurs when the Thunderbeast migrations bring them near us. Even if its because 'humans' don't rank highly on the threat index of a death world as you say, the policy of doing things in pairs is a pretty clear indicator that two humans are more than a match for a single spider, so why spiders? Or why not pairs of spiders?