It's not that they're not powerful, it's everything else. Spreadsheets are great for quickly putting down some data and evolving your understanding as you go. Make a pivot table, filter it, make some charts, show it to someone, throw it away. Where they come unstuck is on long running more or less static important calculations that everyone uses. This is because 1) no version control, so you have copies upon copies being passed around. 2) inscrutable formulas that should be documented udfs. 3) data with no constraints like types, nullability, foreign key, so the data has no integrity. 4) insufficient tests. 5) insufficient error messages. 6) insufficient logging. Etc, etc.
I sort of see where you're coming from, and it's probably correct that processes that rely on complex spreadsheets would often be better served by well engineered and designed software. But that is often not the realistic alternative .
The realistic alternative would be a mess of poorly engineered custom scripts, which offer no advantage over the spreadsheet.
With the spreadsheet, anyone can take out their calculator and check the results, without knowing any programming. That's really the killer feature.
