The latest and greatest PC components (the fastest CPU, the fastest GPU, etc.) all hog power, all run hot, and all need hefty heatsinks for cooling.
When was the last time you see any computer manufacturer squeeze a heatsink the size of a Thermalright XP90 into a 5-pound laptop? Or when was the last time you saw a notebook with a 100-watt-peak CPU like a Pentium-4 last longer than 1.5 hours on the biggest, heaviest 2-pound Li-Ion battery (considering that the best 9-cell Li-Ion batteries out there right now might be able to provide only a maximum of 70 watt-hour of electricity)?
Figure that out, and you have your answer.