This has been true whenever a society has injected large amounts of capital into the bottom classes of its society.

Slavery, colonialism, and cheap imported labor are ways that lower mercantile classes can see significant wealth (generated from slavery, the colonies, or cheap labor) injected into their class at very little expense. However, they are not the only ways. Expansion into new territory, gold rushes, new technology, and governmental policy are among a very long list of other things that can have this same effect.

The most important one for a society, however, is the fact that sufficiently progressive taxation and societal reinvestment can do this as well.

This is one of the primary reasons why democratic capitalistic states tend to have better overall growth than more strongly state capital societies. And more controversially (and far too complex to talk about here) why socialist countries were often able to industrialize in under a generation.

When hard work can't guarantee a better life, it is a sign that a democratic society has hit a degree of wealth inequality great enough to translate into a failure of democratic society to represent and act on behalf of the interests of the majority of its population.

The fact that this is the historical norm is a reflection of the history of authoritarianism (monarchism) in human history, not some unwritten rule of physics.