There are a few reasons.

The first is that, actually, there are cemeteries everywhere - every time a new rail line or highway is added in England (which requires digging up a miles' long line of land), they find multiple cemeteries from throughout the island's history.

But the number of cemeteries we find is limited by a few factors that reduce the final tally considerably. The first is that, in many cultures and during many periods of history, people didn't bury their dead (or didn't bury them in ways that are easy to see). It's likely that most people in Iron Age Britian (1st millennium BCE) were cremated and buried without urns (or had their ashes scattered), meaning it's basically impossible to find any traces of their bodies. Many people were buried in cremation urns in England's Anglo-Saxon period (5-7th centuries CE), but these graves were usually very shallow and have frequently been destroyed by 1300 years of plough damage as people forgot about the cemeteries and turned the fields into farmland (it's common to find broken cremation pot shards - how many cremations have been destroyed like this is next to impossible to say). Add to that the fact that many types of soil are very good at decomposing bone (especially acidic, sandy soil), and a lot of bodies simply decay without a trace. Some early medieval cemeteries have only been discovered because the metal jewelry and pottery buried with the bodies survived; all the bones are gone. Because, for many periods of human history, people weren't buried with durable objects in their graves, these grave yards can become completely invisible.

Despite this, it's incredibly common to find human remains during any kind of diging in Europe. While excavating an early medieval palace last summer, we found 6 Bronze Age burials (5 cremations and a full skeleton), a human jaw bone that had been disturbed from who knows where by a medieval plough and washed down into the stream bed, and other traces of burned bone that could easily have come from cremations whose contexts were destroyed.

Here's a map that shows only the cremation graves from c. 450-600 in England that have been recognized and recorded (there are orders of magnitude more that were destroyed by earlier ploughing, inattentive builders, and 1500 years of time and decay).

In places like Egypt where the preservation conditions are better, archaeologists have found Roman cemteries with over a million bodies.