Recently a poster asked why many people have the habit of leaving the last ounce or so of beer in the bottle. I think the reason is that when bottled beer first came out over 100 years ago, the beer was not filtered. A sediment would form on the bottom. To avoid pouring the yeasty lees into the bottle (or mouth), the drinker left it in the bottle. This practice is still followed intentionally for Belgian and certain other beers that are bottle-conditioned. Some people pour the beer "all-in" but many like myself like to separate the clear part from the yeasty sediment. Once the habit became ingrained to do this in the 1800's, drinkers kept doing it out of habit, even after bottled beer became filtered.
Another explanation might be, the beer at the bottom was flat, or perhaps through a kind of contrariness, or wariness, drinkers did not want to be seen draining the bottle, it was uncouth or perhaps was felt to show too ardent an interest in the drinking experience.
But I favor the first explanation above, it seems to make the most sense.