Well then, show me some preceding games with as pretty environment as Oblivion's.
First thing first - what do you mean by "pretty"?
Do you mean aesthetic, or technical aspects?
Because I can grant you that - oblivion looked new when it was new. It nevertheless didn't really look good - indiscriminate bloom, covering indiscriminate specular, veiling already insufficient resolution textures and bumpmaps plus blurry and 'dull' water reflections.
In that case HL2 would probably be a good counterpart. It's not "pretty" as it goes for different kind of aesthetics, but it's an opposite of Oblivion in the sense it actually strove to make the best of its tech and it shown.
If you wanted an open world game, you have STALKER, although it released later, partly due to development hell. Overall a much better looking game, also featuring dense vegetation and bumpmaps, but also having stuff like really accurate lighting.
If you mean just aesthetics, adjusting for older tech, then Morrowind already had Oblivion trounced soundly, before it even started development.
Sure they put some effort into making it look nice
Well, no.
Not spending even the amount of time required to come up with the idea that horses shouldn't have that plastic sheen pondering selectively applying specular doesn't qualify as effort. Neither does just copypasting the same shit over and over to cover 16 or whatever number it was square miles of the game. And that's what oblivion is - a huge copypasta of already bland content. Everything in this game, from minute aspects like application of given kind of special effect to entire geographical regions is antithesis of effort.
The only thing putting effort into oblivion was your comp sweating fucking solder over running this unoptimized turd.
OTOH Skyrim, while never exactly looking new, just oozes effort, at the very least in terms of visual design. Visually pleasing and, more importantly, distinct vistas dot the entire fucking place, and continue to look good with changing lighting or weather, even the dungeons, rightly criticized for their linearity, manage to remain distinct and memorable despite asset reuse.
they remade their shitty engine far more than they did for Skyrim.
Maybe that's because Skyrim had to run on the same antiquated Xbox "red ring of death" 360, genius?
All the more reason to compare them side by side without needing to account for tech progress and while some minute technical aspects, like character model polycount or light source placement are a step backwards (presumably for performance reasons - running a 2011 game on half a decade old hardware), overall the game just crushes oblivion.
I can only think of Gothic 2 as being a nice open world as Oblivion's but it's not as pretty looking (forests and meadows in Oblivion are better than Gothic's)
Better in what sense? Greener? In that case games on c64 had even more saturated colours.
The Dunmer in the screenshots is literally just a human with blue skin.
Have you ever seen a human?