You can't win the all features on time game

It's an interesting assertion that "You can't win the all features on time game".  Why can't it be won sometimes? I feel that developers are hard-coded to always want to negotiate on scope or timelines.  We always ask our project sponsors to prioritize the work.  Why can't we ever look at a package of upcoming work, and just agree that it is possible to do it all in the available time?  Surely our project sponsors don't always ask for the impossible?  But no matter what they ask for we always seem to come back and ask for the work to be prioritised, and some items to be de-scoped or postponed.  Why do we do this?  Is it just habit? Or is it always impossible to deliver the whole scope on time?  Do we negotiate just to feel like we've had a little win?  Or is it to reduce the amount or work we need to do?