Well Daedra would rather meet in being than send surrogates or encoded messages.
Didn't Sheogorath meet the PC in the Mantellan Crux?
Hircine fought the PC in Morrowind?
Clavicus Vile bargaining with Cyrus?
Short answer, because of CHIM. My general idea is that the princes know CHIM when they see it. I doubt they bother with other mortals.
Imagine a massive woodland with a bunch of animals in it. Each animal has their own motives, for reasons unknown. Also in the woodland are a bunch of wonderfully pliable insects. The animals find out they like toying with the insects. So each animal gathers a group of the little insects to toy with. The keep away from the other animals usually, unless feeling exceedingly bored. And there would be no sense in using insects to fight the other animals, because the other animals could simply trod on the insects. So under this agreement, the animals go about their business having all kinds of fun with the little insects.
:D
This is a nice anology, but they shouldn't underestimate the mortals, if nothing then because as noted before, they are the destined bearers of CHIM. See what Vivec did to Azura. I doubt anyone else ever did anything worse to a Prince
And to get myself back on topic, thanks for all the answers, I also think that, between each other the princes talk via avatars mostly.
I think it's also clear that while they do have the occasional spark of cooperative behavior disorder, it seems they're largely incapable of organizing themselves in bigger numbers (say 5+).
In all honesty I asked all this because of a mod I'd love to make one day regarding the continuation of SI
But it's very questionable if I find enough time to try my hand at modding again.
Say, what do you think the other princes would consider the late MadGod? I know they know he's not a Daedra, that's clear, but otherwise?