Because they are controlled by the human player.
If you look carefully though, you can see that the southern Irish province has Aedui as "founder" faction. This is highlighted by the fact that the Eleutheroi AI (if given time) will build an Aedui governmental system as well as paved roads there! Nice exploit if you play the Casse slowly (or if you play expansionist Swêboz/Lusotannan), because of all Barbarian faction, only the Gauls can build paved roads.
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