Part of the problem is that 19th century socialists were fond of the labor theory of value, which wasn’t formally refuted as a valid economic theory until the end of that century, and which even then continued to influence socialist and anti-capitalist thought.
The labor theory of value goes that since all material wealth requires some human input to produce (even an unskilled laborer directing an otherwise automatic machine), value is directly related to a product’s labor content. While the 19th century saw a tremendous increase in the output of material goods, socialist critics of capitalism looked at the misery of the working class and concluded that measuring wealth in material terms was misleading, along the lines of the following argument:
[socialist argument]While mechanization made it possible to produce much more products for the same amount of labor, paradoxically it made those products less valuable for the worker to produce; since those products were cheaper they earned the laborer much less per unit, yielding a zero net increase in the worker’s wages for a day’s work. A craftsman who made sewing needles by hand could only make so many, so needles used to be expensive. Automatic machinery made producing needles faster and easier, but since they now sold for less, the worth of a needlemaker’s time and labor remained unchanged. Industrialization made material goods more common, but anything that required a hour of a worker’s labor to produce would ultimately still cost an hour of another worker’s labor. Compared to a medieval serf, a modern worker might live in a brick and sheet metal tenement instead of a thatch hut, and wear cheap shabby machine spun clothing instead of sackcloth, but in terms of how desperately they had to struggle to obtain the minimum necessities of life, the modern worker was little better off.[/socialist argument]
So according to the socialist labor theory of value, in terms of how much people had to labor to obtain anything worth having, “wealth” in that sense was a zero-sum game. Capitalism was simply a more efficient way for the elite class to direct the labor of the working class into providing the elite with more leisure and luxuries, leaving the workers awash with more junk but no better off in standard of living.
Needless to say this view has been largely refuted. For starters, labor does not automatically equate to value- garbage that required a lot of labor to produce is still garbage. Secondly, even buying into the “hours per day of labor to obtain necessities” argument, having more material goods is desirable in itself, despite what critics of materialism might claim. Thirdly, people in developed countries are truly physically better off; they’re better fed, warmer, more rested, and healthier. When it became impossible for even the most strident socialist to deny that people in the first world were better off due to industrialization, they changed their argument to say that whole countries were better off but only because they exploited poorer weaker countries- the “class” argument was changed in scale to international.