Why do many native speakers say "It is not polite to say 'Thank you in advance'"?

Question

Let say, in a letter, we want someone do something for us in the future. We know that now that action hasn't happened yet.

At the end of the letter, we say "Thank you in advance!".

Why do many native speakers say "It is not polite to say 'Thank you in advance'"?

Some even feel offended.

Answer 1

The reason some people would find that phrase impolite or offensive is because it assumes that the person is going to do the thing you asked of them.

Essentially, you aren't really asking. It also implies that you would retract the gratitude if they refuse.