Because we don't have the fabrication technology to expose a whole wafer at once. Since we're essentially shining light on the surface, the wider we make the beam, the softer the features get, specially towards the edges (because the light hits the edge at a different angle).
There's a sweet spot of size-vs-yields. Trying to make bigger chips requires multiple exposures for the same die, and getting the exposures to line up properly is extremely tedious.

That's why it's easier to make lots of small chips on a wafer than several big ones, and why people are trying to find better ways to communicate between chips...