Those Linotype machines (or similar type-settings machines) you see at the start of the process create lead bars because casting lead takes only a short while. Casting copper in it would be far harder and more time-consuming, especially if the typed line contained an error (those machines didn't have a backspace!)
Once they had the lines of type, they couldn't use those to make printing blocks directly as they are still "negative", and they would melt when brought into contact with copper.