I can see problems with both the question and the proposed answer. The question is a bit like someone asking why they can't just write "Hello world!" on a piece of paper. In the answers, on the other hand, there is an inherent assumption that Excel is always going to be the wrong tool for any task, and has been entirely superceded by R. Almost any ad hoc use case is going to favour Excel, and anything more stable and repetitive is going to favour some form of programming solution. Excel also works well in hybrid situations where both creator and user of the data and analysis both want access to the source data and the ability to create their own analysis and do their own checks, or further ad hoc analysis. I see nothing wrong with an answer of, "In this case, you're right, Excel could do the job easier and quicker, but this is just an exercise to help you learn R. You will soon be doing things that Excel can't."