Why did Marx believe capitalists profit from workers' labour?

According to Karl Marx, capitalists profit from workers' labour through the extraction of surplus value. This process is rooted in the labour theory of value, which posits that human labour is the sole source of economic value.12

In the capitalist system, workers are paid wages equivalent only to the cost of their subsistence—the amount necessary to sustain themselves and their families. However, the value a worker produces during a full workday exceeds the value of these wages. Marx argued that once a worker has produced enough value to cover their own wages, they continue to work for additional hours. The value created during this extra time is known as "surplus labour," and the resulting surplus value is appropriated by the capitalist as profit.123

Marx viewed this appropriation as a fundamental form of exploitation, where the owners of the means of production amass wealth while the workers, who actually create that value, are denied the full fruits of their labour.32