You probably have a much higher culture rating than the civ you are attacking, and if so, now you know why.
In a recent game, by the time I found France and Russia, both their cultures were about double mine. Once I got up on the power curve, I investigated a few cities---They each had continents to them selves, and all the central cities had every available improvement, and the culture items had been there for eons.
When I invaded Russia, I bombarded heavily at first, until I had virtually eliminated her excess military, then I started just taking cities directly with tanks, cav and Mech infantry. By that time I was mostly in the smaller cities, but a few of them had banks, marketplaces, aquaducts, airports and harbors. Never a culture improvement.
By contrast, in the same game, Babylon was in awe of my culture when I invaded him. No improvements in any city, except aquafducts in some. He ceded three cities to me for peace, and none of them had a single imiprovement.
