Why is future opinion better than historic opinion?

In a discussion about slavery and equal rights and such, someone mentioned that in the future we’ll look back on how we treat animals (slaughterhouses, etc.) and realize it was just as bad if not worse than human slavery or segregation.

Slavery and slaughterhouses aside, I noticed something I hadn’t before: we’re prejudiced against people in the past… or at least we’re prejudiced against their opinions.

All other things equal, we assume someone in the future has a better opinion than someone of the past. If Neo in 2110 has a certain opinion about an issue, we assume that opinion is better than Napoleon’s opinion from 1810. And it’s not just because we like Neo better than Napoleon.

So why is ol’ future boy’s opinion automatically better than someone’s in the past or even ours now?

Overall, I think I know why. We assume humans collectively are getting smarter, more intelligent, and simply better overall because you and I also assume we’re individually smarter, more intelligent, and better overall than we were, say, five years ago.

But is that last part a valid assumption?

And even if it is, can we generalize our individual experience to humanity as a whole?

Most importantly (or at least most interesting to me), why?

Or why not?