In the airline case, FAs can write these notes during the times they have no other duties on board. It's a nice touch, but it doesn't require much cost or effort. I'm sure marketing has studied the impact of these notes versus having a FA come to your sear and doing a personalized verbal thank you at the end of the flight. OTOH, there have been tales here of individual FAs, pursers, and pilots taking the initiative to do the handwritten thank you notes years ago; some crew became famous for these efforts.