Any wafer fabrication process calls for a (usually large) number of different steps, and these will be somewhat different for RAM, logic and specialised high-speed functions such as video and USB. Those functions could be manufactured on the same wafer with moderate compromises, as is done for microcontrollers, but power management functions such as high power DC-DC converters require significantly different fabrication and would be impractical. By analogy, you can make most parts of a motor car from sheet metal but you’d struggle to make an engine that way, not that it would be technically impossible but certainly not practical. Besides, much of the function of a motherboard is to house the connectors, so any monolithic solution would need something analogous to a motherboard even if it only had one chip on it.