Depends on the game, and the 'engine'. is a game designer doesn't want to create their own engine they often buy an Software developers Kit of a main engine and modify it to their own needs. This has a lot of the basic stuff in it. However you always customize all of the scripts etc in it. But generally you create your own art assets because you want to own them. You 'can' get some generic stuff but you would then need to tweak it. A good 3d model artist will re-use some things but you need to customize/tweak things as the game adapts.
Xcom EU/EW - is using the unreal engine. Which is odd for the style of game but that was a decision they made. Then they customized all of the things they wanted to make it fit. This saved them probably a 1 or 2 of time in making an engine while coming up with concept art etc.
DF is more complex than many games on the market. The amount of real time decision's being made on so many levels is HUGE. Toady choose to focus on that over graphics. The community has stepped up to meet their needs for graphics within the feasibility of what they can.
There are many changes being done and DF is technically still in alpha, as Toady plans to make this his life's work. With changes being done all of the time, any mods needs to be simple enough to keep up with those changes.
Now go back to what Dirst said about the 'number of models needed'.
Why do you think games have only a few models for things like cars. Hey look there is really only 10 car/truck models in like X-Com, they are all re-skinned with 10 different colours. Because the time to make 100+ car models isn't worth the time/effort in a game in which they are technically 'background' props. Then they had to add in animations for each of them to 'explode' and get parts destroyed. Do that with 100? No way, make another 10+ maps to play on. Yes, that was a much better decision when they released the DLC.
I was reading about when they wanted to add in the Mech for melee move in EW and and the Lead App Design person said she wanted a custom animation for killing every alien in the game. The design team said "you get 3" because they knew they didn't have time. In the end they came up with 1 super cool move on what happens when you use that effect, (and it varies based on height/terrain and what the alien get's hit into). This is a AAA company, and they still lacked budget/time for it because what do you focus on?
DF has amazing world generation with a rich history on top of fun mechanics in which each time it's different way to get dwarves killed.
I'm old school. I played the first early MUDS on the internet back college (when the html part of the internet was being born) Immersive gameplay is where it is at. We had huge fun and we had to read combat logs when fighting as stuff spammed by in real time. Kids these days are spoiled with fancy graphics with repetitive static gameplay.
Just having the tilesets make it fine. Stonesense breaks the "hard to learn" building barrier.