We can't, because if we travel into the future from this point in time, it will be the future of this reality, in which Biff is corrupt and powerful, and married to your mother, and in which this has happened to me.
20 seconds sound clip from the Back to the Future Part II movie soundboard.
You can hear this line at 00:54:38.367 in the Blu-ray version of the movie.
Quote context
[...]
- Son of a bitch stole my idea. He must have been listening when I...
- Right. So we go back to the future, and we stop Biff from stealing the time machine.
- Bulletproof vest! Great flick! Great frigging flick! The guy is brilliant.
[...]