First you have to understand the difficulty in making a very fast computer:
GHz is billions of Hz. A billionth of a second at the speed of light is about 11 inches. So to expect computers to break the 4 GHz barrier of frequency, light and any electrical signal moves only less than 3 inches.
So you see that the tighter the circuits can be made, smaller is faster. But to get billions of transistors together on a 5 mm square silicon chip, fantastic techniques must be used. Inscribing them with light can be done down to 23 nm or nanometers (billionths of a meter) trace widths, ultraviolet light must be used. That has the shortest wavelength and can be focused to the smallest image for the most accuracy. Only X-rays are shorter, and that use is only experimental so far.
Changing the width of traces to something smaller to make chips more complex without getting huge and slow is very expensive. Nowadays a CPU costs many times what it used to. Any more expensive, and the market will dry up. Not a good business model to invest in making them.
Laser optics designer. Electronics firm manufacturing engineer. Optics designer for the last 55 years.