User blog comment:The Milkman/What I learned from Skyrim/@comment-1786741-20111211032644/@comment-4644719-20111211042500

If they can sense when I am there, then why can I sneak up on them?

Some Draugr do seem to wake up when I pass by, but most of the ones seem to be alive(undead?) and walking before I get there.

My (flawed and partial) understanding of the Draugr is that they were the willing servants of the dragons. I'm speculating here, but dragons may have given them immortality for their services. With the dragons mostly dead, the Draugr are also mostly dead.

Such servants of dragons would have been thrown out of society, possibly forced to occupy their old forts. Bleak Falls Barrow was built in the times of the dragons according to Farengar Secert-Fire. In time, these old forts would become tombs as the bodies of the dragon servants deteriorated.

Anyways, that's as much as I can piece together, it's not even definitive enough to put in a wikia