In addition to what has been said, tooth decay is also mostly a modern problem. Not that it didn’t happen before, but it’s generally the result of eating lots of sweets and sugars, which we have access to in abundance, but most of our ancestors rare ever had. So those old skulls mostly belong to people who rarely or never ate sweets and thus had very little tooth decay.
A lot of this comes down to survivorship bias. Those skulls with teeth are found *because* they happen to reside in some set of conditions that aid their preservation. Many remains decompose far more significantly, and may do so in a relatively short time frame. Of course, since those are very damaged (or more or less gone), they don’t generally receive the same attention (as interesting specimens, for instance) or are overlooked entirely.
The teeth in your skull are being constantly assaulted by bacteria and acidic materials that they produce, or you consume, a daily onslaught that your body has to ward off.
Conversely, a skull in some dry dusty cave or buried in the desert may experience very little adverse conditions. The materials in bone and teeth themselves are fairly tough, so without some active effort against them, a lot of material can survive a long time.
Contrast that with a skull buried in some more acidic soil, or out in the open. They can decompose in a matter of years, be damaged by animals, crushed, battered, and so on.
The one that’s going to make the “hot new museum display” is going to be the well preserved specimen, rather than the pile of shards and teeth.
TLDR: Many don’t, but sometimes sitting in some quiet, dry, out of the way place for thousands of years is less damaging than being exposed to junk food and soda for a few decades.
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