[chris (OP)]

Is this an intentional aspect of mirror design? Why does it happen?

[Geezer]

errrr, I think you got it downside up.

The text on the mirror should say "Objects in mirror are closer than they appear"

It's because the (usually) nearside mirror is convex to increase its field of view. The idea is to make sure you are aware of any vehicles that might be sneaking up on your "inside" before you move back into the "slow lane".

[imatfaal]

The Geezer is right.  I offer this incontrovertible proof - well actually it's just a movie still
 [ Invalid Attachment ]
I still cannot embed pictures in the post without the workaround

[Phractality]

The image you see in a convex mirror is called a virutal image. The virtual image, which is located behind the mirror from your perspective, is closer than the actual object, so our eyes converge to a point which is closer than the object. If we could judge distance by convergence of your eyes, you'd think the object is closer than it actually is. However, the angular size of the virtual image is smaller than the angular size of the object. We tend to estimate distance by angular size rather than by the angle of convergence of our eyes, so the think the object is farther than it really is.
 Image source.

[SeanB]

This as well....