They could. It certainly isn't impossible. But without a cartographer manually positioning each one, that would give rise to other problems to solve, to prevent overlaps with other artefacts and markings such as taxiways and holds. So, there's a cost to doing such a thing, only for those airfields large enough that the runway numbers end up very small when fit on to a page.

How would you propose we do that without the numbers spilling over the sides of the runways?