Saying thank you to a plant for growing you a fruit is strange behavior. Saying thank you to a LLM for growing you foobar is also strange behavior. Not doing either is not degrading behavior of the grower.
https://www.agrieuro.co.uk/robot-lawn-mowers-manageable-with...
LLMs are bound to change society in ways that seem strange to people stuck in outdated contexts.
Saying thank you to a LLM is indeed useless, but asking politely could appeal to the training data and produce better results because people who asked politely on the internet got better answers and that behavior could be baked into the LLM models.