We found plenty of Dwemer schematics in Morrowind. The Dwemer machinery still works and everything. The other races just can't make heads or tails of it, and thus it remains unused. Also, technology has been pretty stagnant since the beginning of time. There's been very little technological improvements.
That's because the Elder Scrolls is medieval-style fantasy game with armor and swords and wizards and trolls and goblins. If you bring too much technology in then it stops being that and starts being something else. Too much technology would ruin the series for a lot of people. It's two completely different styles.
Also, I'm sure they could come up with a lore-related reason as to why technology pretty much never moves in any expected direction.
and assuming each title's timeskip is as long as Skyrim's
The leap between Oblivion and Skyrim is the largest we've seen. Between Arena and Oblivion, it's only been approximately 50 years.