Lots of folks don't remember (or were never taught) that concrete blocks are crazy strong in the correct orientation (with the holes looking up) and have almost no strength at all laying on their sides (with the holes pointed sideways). They also have a top and a bottom-- the webs are fatter on the bottom and taper to the top. Concrete blocks sitting on footers hold the entire weight of a house, and have side loads that are crazy from the dirt pushing horizontally against the block.
They will certainly hold up a car.
The problem comes in that nobody is ever satisfied with one block high, and they dry-stack 2 or 3 of them, generally with zero thought regarding the proper orientation or how the blocks are fastened together. They are also not very wide (8"), so they tend to be really, really prone to tipping over when stacked into a tower.
Blocks need to be protected from sharp corners, which can start a stress riser and crack them easily. A dimensional lumber block sitting on top of the block (like the sill-plate on your house) protects the block.
Once you know that you need go to all that trouble, you start to see the wisdom of just buying jack-stands.