1. They don’t have to, I have Gan chargers that do a lot.

2. Less pressure on laptop manufactures to shrink as much.

3. Duty load: the big one.

You’re not just charging an hour at 20w and unplugging it, or tapering the charge, you could be using it at max rated output for days, that means much more stress, it needs to radiate more heat, and generally needs to be bigger.

As for fluctuating load, shouldn’t be as much of an issue, laptops often do their own power conditioning because the battery is fairly rough, and you’re going from 12-20v down to 5 and 3.3 then 1.6 and 1.3 to vcc for the chip, there’s plenty of filtering at each stage and they’re isolated by the smps controllers and input caps.

But pulling constant rated duty cycle basically doubles the size of power supplies normally, GaN technology helps a lot (which is why those little power bricks can do 100w+ now even though they got a lot more dense).