I've often wondered why so many people (myself included) can re-watch these old episodes of UM so many times and they never get old even though we know some of them have been solved and we already know exactly what happens.

For me, it's Robert Stack. I love his voice and storytelling.

Yep, also pacing, storytelling, atmosphere.

The nostalgia and atmosphere keep me watching it over and over. Robert Stack's narration is golden. He had an air about him that was spooky, mysterious and cool, and I think that's why a lot of us love him. The fact that a percentage of the cases are still unsolved keeps a lot of people coming back for more as well. Sometimes I find myself going back over cases that are solved, because to me, it still kind of feels real and feels like it just happened yesterday.

I would cite the production values as a reason that makes the show re-watchable. The show had a great look and feel to it during the late-'80s and early '90s. Although the show went through some tweaking afterwards, in my humble opinion the show retained good production values until the end of its production run. But in general, the show looked more credible as an authentic documentary series in the late '80s and early '90s.

The show is basically a perfect recipe. The true stories, the music for dramatic effect, the recreations, the various theories on each case presented, the perfect host etc etc. Also for many of us it takes us back to our youth. I remember spending many of nights being intrigued and horrified by this show. It's much like an addiction

It's real.
It's (usually) unsolved.
It's scary/dark..

It's intense, it's dramatic.

It's got everything to capture your attention, and the fact that there's so many segments can keep them 'fresh' so to speak.