Because it is far more clever than anything I could do in Excel. And it doesn't require me to duplicate information all over the place. I'd rather have just one master source of data that I do all the calculations on.
If you're doing deep analysis on large historical data, then it's in essence a MIS system, and most businesses do (and should do) that outside of Excel.
But what the article is about, in a growing startup or a changing company (say, completely new product line) the 'one master source of data' is not in a database but the assumptions in your head; and any clever model based on your historical data will be either impossible or far more wrong than a simple order-of-magnitude calculation done on a paper napkin. And, well, Excel is more convenient than the napkin.