Because the definition of poverty is subjective. This is not hard. It's definitely not physics.
With a few exceptions for the homeless and mentally ill, the material quality of life of most of today's poor people (in America) would qualify as well-off a century ago. A middle class person today is obviously materially richer than a 1-percenter a century ago, except for a handful of metrics like house size or number of servants.
But the 1 percenter a century ago had a worse diet, worse healthcare, wildly subpar transportation etc.