A basic overview of the cycle can be explained starting at any point. Igneous rocks cool and break apart into sediment. Then through a number of different methods those sediments compact into sedimentary rocks. Under intense heat and pressure, these sedimentary rocks can become metamorphic rocks. Once the metamorphic rock is exposed to underground heat long enough, it melts and becomes magma. A volcanic eruption exposes the lava, and it cools to igneous rock, right back where we started. The cycle can take anywhere from thousands, to millions of years.
As for why Pre-Cambrian rocks are harder to find: Each time through the rock cycle, when the rock is exposed to extreme heat and pressure, the mineral composition can change. Little by little, the mineral composition changes, maybe minimally at first, but if the rock went through the cycle 5000 times since it was the Pre-Cambrian rock, it would bear little resemblance in any way, visually or structurally.
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