At high end production nodes it is impossible to get an entire wafer free of defects. Chips, let alone wafers, already include circuitry to disable parts of themselves if those parts have defects. Cerebras must have spent a ton of effort on getting this done for a full wafer. You also have problems like variability at the wafer level which you're less sensitive too when you put thousands of chips on a single wafer rather than just one, since they cover a smaller area.

Look at how successful AMD's chiplet strategy has been. Chiplets sidestep the yield problems. Wafer scale amplifies them hundred or thousand fold.

Nothing in the industry is designed to work with wafer scale products, so everything has to be custom made. Yes this is a chicken-egg problem, but it's going to be expensive to get any sort of momentum. The silicon industry is extremely conservative.