Why is it that if you pull a loose thread on a garment it will undo the whole seam, hem, buttonhole or whatever, yet if you actually want to undo a seam or hem, you will have to cut each stitch individually?

Because, if you look at your clothes it's double stitched. If you have a sewing machine you can see that there are two strings of thread entering your clothes one from the bottom and one from the top. So when you want to take a seam out you have to cut because of that double stitching. A loose string is easy to pull out because more than likely the stitch on the other side is already broken.

The bottom thread is from the bobbin. It is what the top thread hooks onto to sew the cloth together. Remove the bobbin thread and the other thread removes easily. If you look closely you'll see the little loops left behind when the bobbin thread is removed. Finally, a topic I'm familiar with.

Also, many mass-manufactured clothes these days are made with serging. I believe a serged seam unravels rather easily. Far more easily than a seam sewed with a sewing machine. And, well-sewn sweaters are REALLY easy to take apart if you know which string to pull! (And I'm not talking about unraveling the sweater, though that is also fun. I'm talking about taking the pieces apart.)