This question misunderstands the nature of surplus profit. In market economics, there is a minimum price that a producer can accept for their last item they produce, to remain in business. That number is different between the first and the last item. The integral of the sale price minus this price, over the full production quantity, represents a surplus profit above what is needed to say in business. There is a similar surplus in each worker's pay -- how much they would be willing to accept to work their last hour, or any hour, vs what they actually are paid. This surplus pay represents a surplus over minimal pay to barely choose to stay employed.
These two dollar values combine to represent the intrinsic "surplus" of a business. The relative negotiating power of the workers, vs. the owners, determines how much of this surplus is surplus pay, vs surplus profit. At the time Marx was writing, in the late 1800s, there was a massive oversupply of industrial workers, and this gave business owners such a massive negotiating advantage that basically all of this surplus ended up as surplus profit. Marx assumed this would always be the case, based on Malthus predictions of unending population growth.
In subsequent decades, the vast oversupply of industrial workers did not continue, more factories were built much faster than workers reproduced, and at least in Europe and N America, worker wages grew dramatically. Workers were able to capture surplus wages for themselves, decreasing the amount of surplus profit.
Sweat shops in the 3rd world today are basically repeating the dynamics of that robber baron era. Globalization is behind the drop in effective wages for labor in N America and Europe. We have not returned to "all surplus goes to ownership" of the late 1800s, but globally businesses capture a larger fraction of this excess as surplus profits today, and workers capture a smaller fraction as surplus wages, than they did 40 years ago.
A Marxist perspective would postulate that the sweatshops of the 1800s, and of the 3rd world today, represent the typical operation of capitalism, while the well paid period for workers of N American and Europe from the mid 20th Century to today, is a transitory exception. And that sweatshops, dirt wages, and massive excess profits for a small owner class, are a morally intolerable situation, which anyone with a functioning conscience should want corrected by governmental intervention.
In general, free market advocates base their advocacy on the empirical observation of Adam Smith, that markets generally lead to a good outcome for society as a whole, via the "invisible hand".
Marxism looked a the sweatshops, dead and poisoned workers, dirt wages, absurdly rich Robber Baron class, and use of the army to benefit the Robber Barons, of the late 1800s, and say that NO, free markets do NOT produce a good outcome for society.
The dispute between Marxists and free marketers' is over whether the sweatshops, or the last 70 years in N America and Europe, is the normal societal outcome from markets.