I admit I am a bit surprised by how violently our citizenship and military brotherhoods seem to opposed to mercenaries and I am honestly struggling find it entirely believable since it seems a massive overreaction and somewhat at odds with our tendency to use native allies and the like but okay seems like I have to accept that mercenaries are out.
Here are the reasons why this would happen:
1) Unlike native Italian levies, mercenaries are likely to compete directly with our citizenry in the roles of hoplites and their aristocratic cavalry. Recall that in this society, war is a fundamental part of how citizens of a Hellenic
polis express their
arete and manhood. Hiring a bunch of foreigners to do it for you is
at best embarrassing on some level. A city that's faced defeates may do it, and after a major war shakes up the Mediterranean it may become more common, but right now it's not.
2) There are probably
countless examples of mercenary bands disrupting democracy. Mercenaries are if anything probably more likely to be loyal to an oligarchy or a tyranny than to a democracy, because while a tyrant or oligarchy may stop paying them, they're probably not going to do so on a whim when they're counting on the mercenaries for logistical support. Ancient Greek democracies tended to be... fickle.
3) Eretria, as pointed out, has a certain amount of martial pride. Despite warnings that our phalanx isn't really first-rate, we've never actually had that experience of going up against a real powerhouse like the Spartans and
losing. Whereas the Eretrians have repeatedly routed the Italian native populations, and held their own against the Syracusans and the Tarentines. As long as that belief in the valor of their own arms persists, the Eretrians have little incentive to even start thinking about why they would want to hire mercenaries.
...
Moreover, it is at best
DEEPLY ambiguous whether hiring mercenaries is even a good idea in the long run, in any era of history. Easy availability of people willing to kill for money, who are as good or better at it than anyone else in the city, has a lot of drawbacks. And there are a lot of dangers associated with having a large army sitting around and likely to explode if their pay is interrupted.
It's not at all clear to me that a regular practice of hiring mercenaries would be beneficial to Eretria, compared to other things we could do with the money that would in the long run strengthen the city.