Do you thank people when you want something from them? Or when you get something from them?
The phrase “thank you in advance” is an accepted technique for encouraging recipients to act favorably on your request. But it’s not the same as acknowledging their time and effort on a task. The latter type of “thank you” conveys appreciation, not anticipation.
A quick, polite expression of gratitude is rarely wasted. It can even double as an acknowledgment of receipt — “thank you for the file.” But it also conveys your professionalism and competence.
Perhaps some correspondents fear that saying “thank you” conveys a familiarity or an informality that does not really exist. Or perhaps they think that saying “thank you” makes them appear vulnerable, that it incurs a debt that the other person can now hold over them.
If so, the issues underlying those concerns won’t be corrected simply avoiding a courtesy. So they might as well thank them first and then deal with the real problem, whatever it is.
In my own experience, thanking someone after the fact is more effective than thanking them in advance. By thanking someone before they do something, I might get that one result, but by thanking them afterwards I stand a better chance of getting even more results in the future.