All good points.
There is another also, or at least an extension on these points.
In an effort to compete, nVidia has pushed the limits of engineering margins to an extent that caused overheating problems, which provoked a counter response from ATI to do something similar.
Intel's manufacturing technique is superior at producing high speed transistors, and with very high IPC on their processors, they have very comfortable margins for engineering their stock clock values while still producing highly competitive products. AMD's current 45 nm process doesn't reach the same speeds as easily (though both can produce 4+Ghz), but as AMD's IPC isn't as high, they already assume somewhat lesser margins than does Intel.
When circuits are more complex, the upper speed at which they can be clocked is limited. A crude example goes like this: Circuits which perform simple tasks may require as many as 200 to 300 transistors to work in unison to complete a task, like addition in a general CPU. Counterparts in GPU's, on the other hand, perform more complex work with multiple operands, requiring perhaps ten times as many transistors to complete the work, and as each cascade of circuit complexity completes their transitions from one logic state to another, more real time is required to complete the work, making the upper clock limit much slower (while the total work accomplished is actually much higher).
Standard CPU's are expected to perform work analogous to:
c = a + b or c = a * b
Whereas GPU's are expected to solve
x = xa * ma + xb * ma
y = ya * mb + xb * mb
z = za * mc + zb * mc
As a single operation.
The collection of circuits required to complete the former are much simpler than those required to complete the latter, and that's exactly what is meant when sLowEnd said they're different architectures.
Even the layout of a circuit can cause changes in the upper speed at which the whole circuit can operate.
If memory serves, the total transistor count in the modern CPU's hovers around 1 billion, while some of the GPU's approach 3 billion.
Edited by JVene - 6/11/10 at 7:16am