We just released this Intel® Core™ i7-2920XM Extreme Edition that runs at 2.5GHz. The real answer to your question is that with a mobile processors we have to be very mindful of both battery life and heating issues. A laptop with a 95w TDP processor would give you 3rd degree burns on your legs are you used it for the 15 minutes that the battery would last. So in order to save power we clock the core clock speed on mobile quad core processors down a good bit but if you check the turbo mode speed you will find that when you are doing 1 or 2 things that arent multi-threaded the turbo mode speed can reach all the way up to 3.5GHz on this processor.
When you go portable you wind up sacrificing processing power. Laptop CPUs are generally clocked lower to save power, allowing for a longer battery life and to reduce heat. The laptop form factor does not allow for very good heatsinks on the CPUs so you can't clock them as high as a desktop without running the risk of overheating, especially with the quad core CPUs. The quads also use a lot more power than dual cores so clocking the quads as high as the dual cores would cause a significantly shorter battery life.