In 1928, Otto Frederick Rohwedder invented sliced bread. Now, bread could be sold pre-cut, which could not have been done before, because manufacturers though the bread would go bad. It was advertised as "the greatest forward step in the baking industry since bread was wrapped." That's where the expression "The best thing since sliced bread" comes from.
Betty White was born in 1922, meaning she was born before the advent of sliced bread.