It's not really the 'saying thank you' that is the issue (although good manners are sadly lacking and "Managers" particularly should be more astute).
It's the perception that the simple bus-drivers up the front are there to serve me; I should have automatic access to that seat when I want it and what's the big deal anyhow?
It's also awkward having to tell somebody to stop talking when we're busy (they can't usually hear ATC) and the general clatter of newspapers, sniffing etc is sometimes intrusive when there are enough intrusions already.
It's not a big deal but I think it's the assumption that the mere workers up the front are there to serve me,me,me I find offensive and that lack of thanks is just symptomatic of that general attitude.