It's the place, it's great to expand there.
Pontus: Takes the small-asia, that includes the Seleucid lands.
Armenia: expands south (since there is little in the north) to, you guess, Seleucia (and in my games they are often in war with pontus too)
Parthia: Expands to the west to, indeed, Seleucia
Egypt: Antioch is a great way to make money in the middle east, just like Tarsus, and those citys are of.... Seleucia
So that is why the seleucid lands are taken so often. At the same time, Seleucia doesn't have über units in the beginnings. The militia hoplites can stand against eastern cavalry, but are being killed by Egypt's Numidian spearman, Militia Cavalry is killed by horse archers, while greek cavalry is killed by spearman (and the AI is often foolish enough to charge them into spearman). Pontus, i think, and egypt have more superior archer units than Seleucia (Bowmen have a better range than skirmishers and seleucid bows are in the second archery range)
That is why I think the AI Seleucid loses from AI 'Other factions' so often