Because they will only be in the state for a short period of time
Tree rings are annual growth rings and their thickness indicates how growing conditions were for that year. Index fossils in their type and frequency are indicative of the environment that those organisms grew in and how prolific life was then in that environment. Ice cores drilled through glaciers and ice sheets like in Antarctica tell us about such things as the concentration of greenhouse gases like carbon dioxide (as bubbles in the ice layers) and also about climatic conditions in glacial and interglacial periods.
