As we have seen, the upper cavity of the torso corresponds to the thoracic vertebrae, and their individual rib attachments.
“Is it just me, or does there seem to be a lot of bony structure associated with the thoracic cavity?”
That’s pretty much the physical make-up.
It, then, should come as no big shock that most of us humans are rather inflexible in the thoracic spine. There is just so much bony material, all of which is attached to the rib cage (which is attached to the spine on the back, and the sternum on the front).
“How to get bendy there?”
Get out your collection of Gratz barrels and put them to use!
You will definitely know when you have affected that particular muscular region, upon regular, and deep investigations into the mysteries of the extension of the spine.