Why do some microchips have a mirror-like surface?
These are probably chipscale packages.  So you're looking at the top of the actual silicon die.  The bottom is processed so that there are solder balls for connection to the PCB.

If you've ever seen a silicon wafer you know that it's polished to a mirror like silvery-gray finish.

One issue you can sometimes have with CSP devices is that they can be sensitive to light, since the silicon is exposed.