My pedantic answer would be "because MS doesn't made laptops"
My real answer is that they aren't specifically better, but the provide a different/better/easier (YMMV) user experience. The touchpad alone is worth it, because gestures have changed the way I do things, and I *rarely* ever use my mouse except when I'm booted into Windows and need it because the Bootcamp drivers don't do inertial scrolling or semi-sane two-finger right-clicking.
You also get wicked battery life and the form factors are industry-leading. The screens are nice, and it's not a weak, feeble, or plasticky construction of the chassis... Cheaper laptops from other brands just kill me when you pick them up and they creak or flex. Some say the unibody design helps with heat distribution too, although I don't know how effective it is.
I won't be buying another PC laptop anytime soon as I've definitely fallen in love with the Apple experience thus far. I'm still not sold on the price/performance of their desktops though...