It's really not. I don't think it's a "3/10 HORRIBURU" game like the OP claims (that spot is taken by actually bad games like Fable 2 that don't even do basic things like character movement well) and I understand that there's some enjoyment to be had, but it IS an utterly mediocre game on all fronts except one: it has a big open world.
I've noticed people somehow become less nitpicky about problems a game has that they would absolutely destroy other games for simply because there's freedom to go where ever you want and do whatever you want. The story is shit and a far cry from better RPG stories? Doesn't matter. Combat is clumsy, badly balanced and boring? Doesn't matter. The RPG mechanics are superficial, badly balanced and horribly designed? Doesn't matter. World & dungeon design is repetitive, overly simple and doesn't offer anything too interesting? Doesn't matter. Graphical style is extremely generic? Doesn't matter. Tons of bugs & glitches? Doesn't matter.
What other game gets a pass for doing so many things so much worse than what the best in the genre are doing? I mean, sure, a game like Planescape Torment has clumsy combat as well, but it's easier to forgive its shortcomings because it has such awesome characters, a great story, incredible dialogue, intriquing setting/world etc.
Of course it can still be enjoyable to just wander around, but if anyone wants a well well designed game with an interesting world to explore and a good story to experience with all of it wrapped inside good RPG systems, one is not going to get it from Oblivion. The game is functional on all aspects and there's no single aspect that is completely broken, but none of it is good when compared to actually well done counterparts.
I 100% disagree with everything you have just said.
Oblivion created a world that felt so organic at the time, and while it did not feel as "Real" as Morrowinds world, it did some pretty amazing things.
The graphics for the time were amazing, (though they aged poorly)
The physics interaction was top notch for the time.
The freedom as you said, was and still is quite staggering/daunting.
The combat while not the greatest, was still fun & a step up from early levels in Morrowind.
The Characters were all unique, and ACTUALLY lived in the world. Which is something even newer games like TW3 can't fully compete with.
I could go on, but there are so many memorable moments to be had in Oblivion. I spent several hundred hours exploring the world, and there was still so much to see at the end of my journey. As in, there are entire games worth of content that would be fresh to me if I booted up Oblivion today.
Still, it is the worse out of the 3 "modern" TES games.
Worst of those three though is still like being the poorest of the three richest people in the world.