The faster you want a chip to go, the less time there is between every cycle. It takes time for signals to propagate across silicon. The smaller the window, The less workable area you have. An entire CPU wants to get clocked together, meaning you want all the components more or less running at the same speed, so they can work together efficiently.

At 10ghz with speed of light delays you can only move 2cm per cycle. And the propagation rate of electrons in silicon is even lower.

Other people have already talked about temperature, so I won’t.