Remember that before we had Saba as a faction the Arabian desert and south arabia rebelled to Parthia. The culture in the upper Nile regions is more similar to the Sabaeans than the Ptolemies. We'd rather have eastern buildings and portraits and names than Greek ones. So it's a no brainer to us to make those upper Nile provinces Sabaean rebel sub factions instead of carthaginian or ptolemaic. THe other stuff MAA hit on the head - it's RTW AI. We tried making the desert areas harder to cross, but the Seleucids and Ptolemies like all factions go for rebel lands first, and so that's why they love coming into Arabia for no good other reason. That puts them at conflict with you of course more quickly than we might like.