Well OP, you make a good point, although I don't think it's feasible for the developers to create
too many unique architecture styles and scenery items. All architecture and scenery must be created by artists and programmers from the ground up, so the more there is, the longer it takes to develop the game (and the more expensive it is).
Also, Fallout and Fallout 2 used a very limited number of tile sets and scenery items (and character models, for that matter), far fewer than in Fallout 3. To simulate variety, the developers had to get creative when designing the various areas in the game, and I think they succeeded in making Fallout and Fallout 2's areas feel unique.
That being said, I do agree with you. I would prefer if Bethesda dispensed with the "open world" entirely (how open it really felt in F3 is debatable, IMO), designed cities, towns and locations as nodes on the map, and simply utilized a random encounter system for when players travel between nodes, just as in the original games. That means they wouldn't have to bother with implementing all the largely empty space in between meaningful areas, could focus on creating more scenery items and architectures, and would have more development time to expend on making each location feel more unique, even if it does use the same old scenery.
In the end, I think the feeling of unique areas depends more on the creativity of the developers (scenery item placement, NPC dialogue, lighting,
et cetera), and less on the actual variety of architecture and scenery items.
Still, Planescape: Torment did it best, if you ask me. 95% of the areas were hand-rendered backgrounds, completely unique, with the exception of insignificant things like generic houses and a few other areas.
Morrowind was like 1% the size of DaggerFall
Daggerfall's areas looked much the same everywhere you went, including the huge dungeons (which, as veterans will note, are easy to get lost in because everything looks the same), and the game made prolific use of random generation. Of course it was a lot bigger than any of its successors. Quality over quantity, in my opinion.
Generic scenery and random generation are an aspect of old-school cRPGs that I don't miss, and never will. I've always hated Roguelikes for that reason.