Before a microprocessor can be manufactured, it has to be designed. This is no easy task. It takes a team of up to 600 engineers. The engineers face a task equivalent to trying to design a small city from the ground up. How much area of the chip should be set aside for temporarily storing information? How much area should be set aside for maintaining instructions currently being used? How much area should be dedicated to accepting information?
Once the areas of the chip have been mapped out by purpose, the circuitry has to be designed down to the individual transistor. With over 500 million of them in modern microprocessors, that's a lot to keep track of. It's like building a city by designing every room in every home and building before you even pick up a brick.