Hard to do wafer scale. At high densities, wafer defects are more common. One of the advantages AMD has over Intel is that chiplets are smaller than whole die CPUs. Essentially chiplets mean get to cut out defect areas of the wafer and still get plenty of useful chiplets. The odds of getting a defect free chip diminish rapidly as the area of the chip increases. This is also the reason Samsung makes great OLED phone screens but could not make a cost effective large screen OLED TV with the same technology.