Consider the plate tectonics unit?

As plates interact and magma is released, new bits of earth are formed. However, the proportion of the earth that is land compared to water doesn't really change much, meaning that for every new bit of earth, some other bit is being destroyed (to keep the quota stable).

This is exactly what happens and old bits of crust disappear and new bits appear all the time. It's almost like the earth's crust is recycled. As part of that recycling old rocks disappear. To find a very old rock, you need one that over billions of years has always escaped the ravages of plate tectonics. And that's rare.