You have to admit they 'only' lost when the Romans stepped in and said "enough!"
And it still took a good while before they finally broke up. And Egypt had pretty much been the whippingboy of Seleucia on land. Sure Raphia was a victory but the war was a loss.

And the myth of the Seleucid manpower is not true. They had massive armies up until the end. And they included Arabs, Macedonians, other Greeks, Persians ect ect... The phalangites were Macedonians, true, but cavalry and light troops were 'natives'.

So in terms of the game the Seleucids should at least be able to hold back two-three of its four enemies at any one time. And it should be able to beat any one of them one on one. As it is it can't even beat Pontus (how often doesn't it lose Tarsus to them?).

The strength of Egypt really lies in its chariots. It has been proven that chariots are overpowered in auto-calc, so the ready numbers of Egyptian chariots really do crush the Seleucids. They could be faced with Silver Shields and still roll on the next turn.
This also explains the power of the Britons and why they seem to conquer Gaul so easily.