First, what people are doing in Excel is coding.
Second, they do it because Excel is, thus far, the best environment available for lowering the (many) barriers of entry that are perceived as blocking one from learning to "program".
Therefore, in my never to be humble opinion, we'd also be wise to try to figure out a way of helping build in testing methodologies into excel. I think it wouldn't have to be that hard. Imagine a testing mode or a test overlay that you could toggle on and off, allowing you to identify input and output cells, setting test data and expected output, etc.
The crucial thing is that the best work on this front would have to come from Microsoft, if it has to be in Excel. One possible fix would be a testing environment that reads .xlsx files – build something great, sell it to managers and MSFT will either buy it or, more likely, imitate it.