The Marxist view is that capitalism is inherently exploitive. You said it yourself: the capitalists are "captur[ing] a portion of the labor's true value." That isn't something you say about someone who is doing an essential part of the work.

In the Marxist view, the work just inherently "happens" (like water flowing down a stream) and capitalists can only be like a water wheel "capturing the value" that's flowing past. Of course, this is bullshit, but that is the mentality.