Nowadays most people do think to look for a chip. I know that around here it is common practice to scan for a chip and most shelters have universal readers that can read the most common chip types, Avid and Home Again, and indicate the presence of other chips. As for why it is not mandatory, laws to make microchipping have been fought on the basis of invasion of privacy and too much big brother in your business.
For those who are getting microchips, please register them. Shelters really do try hard to find owners and at least the microchips are linked back to the implanter so even unregistered the shelter can usually find the vet that implanted them. Most of the dogs at our shelter who go unclaimed are ones that arrive with no ID of any sort.