



I'm tempted to say the Void and the sunlight are connected somehow, but that wouldn't make sense considering we see Sentients utilizing space, which would be even more direct exposure to the Sun.

No, not only Eidolons. In the Gara lore entries, the massive Sentient itself would retreat and hide from daylight and only attack at night. The Eidolons, being shards of the Sentient, retain whatever aversion to sunlight it had.