Big big reason: R doesn't alter data without being asked to, Excel does so, silently, sneakily. Anything important should not be done in Excel. https://www.theverge.com/2020/8/6/21355674/human-genes-renam... https://stackoverflow.com/questions/165042/stop-excel-from-a...
People shouldn't do important work in Excel. If it is important, people should be involved who have invested the time in learning something more powerful. Indeed, we could ask they aspire all the way to good practice and store their data in a database and their code in git. But there needs to be a process to verify model correctness no matter what tool is being used and bugs will exist in R as well as in Excel.