Using the space of an entire wafer for one chip would result in extremely low manufacturing yields. Even with state of the art silicon cleanrooms, there will still be defects in parts of the output.

With CPUs and GPUs, chip makers can disable faulty cores and bin them as lower SKUs to get some yield out of it. But if you're using an entire wafer to embed weights, and a speck of dust causes a printing defect that makes the weights wrong, the entire wafer is worthless.