Generally, atoms of elements that have incomplete octet configurations are referred to as unstable and would readily involve themselves in reactions until they achieve their complete electronic configurations.
Group 1 and 2 elements happen to fall under that configuration. With these groups having 1 ( group 1 ) and 2 (group 2) electrons to lose in order to achieve their stable configuration, it is much feasible to lose those electrons in terms of energy and as such they are considered to be very reactivve as they would react with atoms of other elements. In the end, the goal is to have a configuration similar to that of the group 8a elements.
The group 8a elements on the other hand are termed as unreactive or stable because they already have a complete electronic configuration and as such there's no need to go into reactions with atoms of ther elements. This explains why there are very few group 8a elemements in compounds.
