Part of the reason for Egypt's strength is that chariots are so powerful in auto-calc (also the reason why the Britons seem to steamroller everything). And of course that the Seleucids are surrounded by enemies all wanting a piece of her rather than each other.

Luckily I have changed the game a bit so that not only are the Seleucids hanging on more than 30 years into the game, the others have even ended up going at each other (Pontus taking a bite out of Armenia, Egypt and Numidia fighting over Siwa, Greek Cities actually pushed out of Asia Minor by the Seleucids) and Carthage held back the Scipii for about 20 years (even going on the offensive, taking Syracuse from them) and now they are ripping Numidia a new one.