So the question is why?

In the book itself stated that language is always ambiguous and we can never fully control the meaning of the things we say and write. 

For example, why do people always say thank you for no reason so often when don't even mean it?

Well in our very humble opinion being grateful may be seen as a social convention. In reality, it is a reminder that life owes us nothing, that it is a good idea to be thankful for each and every bit of kindness that is offered.

" Love may fail, but courtesy will prevail." - Kurt Vonnegut

In other word, civility. It is the oil that makes the human interaction machine run smoothly. Manners are a subset of civility. People say thanks as an acknowledgement of effort, sometimes real, sometimes symbolic, as with all human interaction.

BUT

Hmm again

Why is it so hard to get a thank you sometimes ?