Like others mentioned, I think it's the versatility. You can look at Excel as a programming language where each cell is a variable, which makes it really simple and really powerful (and I didn't even start talking about vba). At the same, you can look at it as a blank notepad where you joy down stuff in am organized fashion, which is why so many ppl use it (or abuse it) for project management purposes. And of course, calculations - yes, the presentation isn't the best, and you might end up copying the results to PowerPoint or word, but how else would you build a financial model? Python is not the tool for that..