They do this to set worst expectation. It is not meant to discourage people from getting in line, but really to have the guest be prepared. If the line is 40 minutes and something happens to delay it (Sadness needs a longer potty break, group of 8 wants separate photos, etc), then the guest has a cushion in the event they have a Fastpass elsewhere or a dining reservation.
Telling a guest it is 40 minutes when it is 40 minutes and then having a delay that causes that same guest to miss something elsewhere results in complaints at guest relations and emails to Disney, which they get thousands of daily. This is just one way to attempt to minimize that.