Because there's no inflation in civ, 2 gold in the twenty first century has the same value as in 4000bc...
Well there is some sort if had oc inflation, 100 gold is way more precious on turn 30 than on turn 300... And army maintenance increase with time too., as long with buildings maintenance.
I really don't miss the cIV inflation. Visiting the economy screen was sure to put me in a bad mood back then.