Back to the Future had an extremely convoluted time travel theory that didn't actually make sense, but one interesting idea they sparked is that you create branching timelines when you go back to the past. Meaning your present timeline remains unaltered, but you basically skip to a new reality when you time travel. Essentially, they claimed the multiverse exists and you travel across dimensions, not necessarily time, when you used the Delorean.

Maybe this is why we never meet time travelers. Because our current universe is an unaltered world and any time traveling that happens here just sends people to other universes instead of our established timeline.

This theory is kind of nightmare fuel when you consider Doc and Marty left Marty's girlfriend on her porch in a dark future and just expected her to be there when they "fixed" the timeline. Nah, bro. You just abandoned her in the darkest timeline. The girl you picked up was an alternate reality version of her.

*EDIT: Back to the Future, not Bank to the Future.