There's a lot of discontinuity in the movies, but this part in the movie gets it right.
In this part you have to see it as if the past is the future is the past. That which happens in the past would have already happened anyway.
In the rest of the movies see time travel as multiple universes.
A better question would be why Old Biff in BTTF2 returns to the same future he left (returning the car) even though he handed life changing information off to himself in the past.
If it's true that he changed the past, (which is the past that Marty returns to where his dad was dead), then how could Old Biff travel to the unchanged future.
My explanation of the phenomena is that it's possible that we're viewing selected portions of a time travel split in time. That in fact when Old Biff returned to the future he split in two, one returns to the timeline where he's rich, the other returns to the timeline where nothing has changed. However that leaves a problem. If he did split in two, then when he returned to the timeline where he was rich, then there'd be two of him, since the original would have no reason to time-travel because he's rich.