Remember that everything in game development comes with a cost. By working on one thing, the designers are not working on something else. There simply wasn't enough budget or timeframe to create the different era portraits for Civ4. I know, I was there.
This is an important element, and I wish people would remember it during discussions about DLC vs functional multiplayer (entire 1st year of civ V's existence) or how the engine runs the game.
It is indeed very easy to throw in new civ names/UU (modders do it all the time), the art isn't. I'm still frustrated that karadoc basically solo-fixed all the control/UI problems in civ IV mod yet nobody at firaxis bothered despite knowing the issues forever (DoW w/o prompt, moving against one's will, unit selection hotkeys, waypoints).
V prioritized its graphics way too far ahead of how the game actually plays, and I don't mean their design choices (though vanilla was quite broken from a design standpoint, too). We're talking about a modern game where the engine can't keep up with a human being making unit selections with the shift key...that's one of the most straight-up pathetic displays you'll see in any AAA title. Then again, if you ask professional programmers how they feel about the way civ IV and V are coded, the responses are both enlightening and depressing at the same time.
You see a lot of programmers point out major flaws in the way the code was written. How many do you see say "you know...I looked at this and it really isn't that bad. I would expect this to run in a fairly optimal fashion"? I haven't seen that happen yet.
The ONLY quality thing any civ game has is its strategic/flow design. Firaxis sucks at building/using engines, making a good UI, and avoiding serious (DoW without prompt, often without actually holding alt) and occasionally game breaking (unlimited free great people in civ V, unlimited oracle/lib techs in civ IV) bugs.