Because the rocks extruded in a volcano suffer a lot of changes during its way up. Moreover, a mantle rock is composed by many different minerals. When tempera

Because the rocks extruded in a volcano suffer a lot of changes during its way up. Moreover, a mantle rock is composed by many different minerals. When temperature goes up, some minerals melt first. So, you expect the extruded magmas to be enriched in those minerals. The extruded rocks are not sampling the mantle properly. Of course they provide tons of information, but is not the same as having a fresh mantle rock.