Why does the Marxist school of thought conclude that Capitalism is predicated on exploitation?

According to Marx, Capitalism is an economic system founded and predicated on exploitation. By "exploitation," Marx means that the skills and labour of individual workers under that economic system are used and employed for the sole benefit of the bourgeoisie or "ruling class" - the business owners, industrialists, and mega-rich of the global economy. Workers impute their skills or "labour", such as melding, to some kind of base or raw material, such as iron. Following this process, the bourgeois Capitalist takes the amalgamation of his or her labourer's skill and effort and the added value which that has brought, such as melded iron, and sells it for money, offering little return in monetary reward for the worker by way of wages. By increasing the value of raw materials by way of labour, then refusing to pay the worker the true sum of money which they are entitled to in reflection of their labour, the Capitalist is able to create a large disparity between the cost of making products and the monetary rewards that those products sell for - "profit". Hence, the truest and most key component of Capitalism - the search for "profit" - is impossible without the exploitation of the working class by way of denying them the monetary value which they help to create.