Couple of things to keep in mind. The only reason they have that technology is because they effectively cheated. Their emperor knew about the Allagans because he was there when that empire was around. And we can't just ignore magic here because a big part of the reason why Emet-Selch was even able to do what he did as Solus was because of the Garleans low status because of their lack of ability to manipulate aether.
Next, the empire is really really bad at governing their provinces. Like it's incredible even. Pretty much every province we visited under empire control had some sort of almost complete lunatic running it and the native people were starving to death and miserable. This is also by design. Emet-Selch literally set up that empire to be a hot mess so it would cause calamities.
