Because you are paying for silicon, and processes, not transistors. Wafers have a certain cost, and litographic processes can illuminate a certain X mm2 of dies in an hour. If a transistor gets smaller, more of them fit in a certain area.

Granted the machines that make them become more expensive, but that's capital expenditure, which gets amortized as time goes on.

So there are two forces here working against each other.

That is what Moore's Law said.