I thought in Marxist theory the workers' surplus labour created this added value, which capitalists took as profits?
Exactly which ignores that value can be created through trade, capital allocation and scientific inventions. In his world a rich person has money solely because s/he "stole" surplus value from workers.
In other words, capitalism allows for the creation of wealth in two forms: rent-seeking and creation of value. Marx correctly identified rent-seeking as an indefensible form of profit extraction and set out to vanish the entire system around it, including capital allocation and labor incentives, which are two valuable things about capitalism.
Interestingly enough there are many capitalist countries in the world (mostly underdeveloped nations) where value creation is essentially nil and all capital is derived from rent-seeking activities.
The current republican party is a strong proponent of rent-seeking activities (e.g. no minimum wage, pro-industry subsidies and monopolies) and anti-wealth creation (e.g. no government investment in research or infrastructure). This places the country on the path to Third World status, as a quick drive through highways in New Jersey and the rest of the North East shows.