I remember reading about IBM Power chips running at 5GHz years ago (maybe it was https://hardware.slashdot.org/story/07/01/04/1824223/ibms-ne... ).

Presumably they were too expensive (manufacturing, power, cooling, etc.) since they're not ubiquitous. IIRC the "barrier" to going any higher was the speed of light across the chip: running faster than 5GHz would require the output to be causally independent from the input!