GHz is just a measure of how many cycles of work a CPU can perform a second. Intel can do more work per cycle. As a result, Intel is faster.
An analogy, you have a Ferrari and a F150. Obviously, the Ferrari can go faster. But if you have to carry a few tons worth of cargo, guess which one gets the job done first? The slower F150 can do it in a trip or two, where the Ferrari needs to make multiple trips, even if each trip is at a much faster speed. That's Intel/AMD in a nutshell: Intel does more work per clock.