The fuel increase is marginal. You still have to burn fuel to carry that fuel to the next airport. And you have to carry the airframe with capacity for that empty seat. The 100 kg for a passenger+luggage is much less than the fractional weight of the plane required to carry them!

For some numbers, a 777-200 carries 313 passengers, weighs 135k kg, carries up to 100k kg of fuel, and has a max takeoff weight of about 250k kg. 313 passengers and luggage, averaging 100kg, weigh just 31.3k kg. Your ticket therefore pays for your weight, plus 5x your weight in airplane structure, plus (up to) 3x your weight in fuel.

If you don't show up, they're still carrying 8x your weight anyways, so saving 200kg isn't that important.

(PS: apologies for the 'thousands of kg' units. I thought it was more readable than Mg or fully written out numbers.)