Why is it so easy to tell the difference between live and recorded music?

The difference is primarily dynamic range.

DR yes (although there are easily available recordings with DRs of 20, or even better, of real music, and when I play them I still know they are recordings), but other things as well, such as ambient cues.

But most things your want to listen to aren't.

Sometimes my Dachshund is fooled by animal sounds on various albums, and occasionally a solo vocal that comes in, but mostly she just looks at me and lets me know she's not fooled, it's still just a stereo...

Tom

In spite of my dog, I still prefer listening to my system than listening to almost any live event, (the exception being the symphony). 

Tom