I thought they existed to overcome the limits of what humans are capable of. People are only able to move so fast, are only able to lift so much, become less mentally present after so long, etc.

That's not generally true. In some cases robots offer improved precision, but in many cases a robotics-oriented factory takes a tremendous amount of capital to design/create and there would rarely be a business case to build it if human labor weren't so costly (due to labor laws).