just the race vanished, not all the traces left of it, and a lot of stuff was left... ruins, armor, items, etc.
That's true, when people explore Dwemer ruins, if they find any of it, they loot it, since Dwemer artifacts are supposedly very valuable, even though the game doesn't seem to reflect this very well. The Empire forbids trading in Dwemer artifacts, but if there's a profit to be made, there will be those who will break the law, and so smugglers trade in Dwemer armor, which, as a result of this, gets distributed to interested parties in locations other than where it was found, and sometimes may make its way into the hands of bandits or adventurers.
Of course, this doesn't explain why it's as abundant as portrayed in Oblivion, but gameplay mechanics do, Daedric armor wouldn't really be as easy to find as it is in Oblivion either, look at how hard it was to get a full set in Morrowind. It's always important not to confuse gameplay mechanics with lore, because with games, there is often some degree of segregation between gameplay and story, things such as plotline deaths being caused by wounds that the same character could easily survive in gameplay, abilities that certain characters or races are said to have in lore being seriously weakened or removed entirely for game balance or because the programming doesn't allow the ability to be accurately portrayed, or locations being smaller than they are said to be in the lore, the abundance of items that should be rare in Oblivion is also an example.
Well, it's supposedly prized by adventurers. A few more successful bandits and marauders would have it. The only thing that doesn't make that much sense is why it's not as powerful as it should be.
I would have to agree, I always found Dwemer items to be weaker than I'd expect them to be, cheaper too, for that matter, generally, I find that they're just not worth their weight, and this goes for both the armor and weapons and the every day items, granted, they're more expensive than most of their common equivalent, but considering that people are willing to break the law for the kind of money that can be made trading in them (Though once again, this isn't reflected in the game, aside from a few quests where the illegal nature of trading in such items becomes a plot point, no where during gameplay does anyone seem to care about buying and selling Dwemer artifacts, which tends to strike me as odd considering that most merchants who buy ingredients or potions won't even trade with you if you have skooma or moon sugar in your inventory.) I'd expect them to be worth a little more than they are.