Yes, it bothers me every time too. (And I'm a professional linguist, so it's my job to stress out about such trivialities.) The poster is right who said that it's likely to be a politeness-related choice. Of course, it is possible to say "now" without being rude, but it would be oh so easy for an FA to miss the subtlety in a moment of stress. Try saying "at this time" in a rude manner -- it just doesn't work, and you end up sounding stupid.