This is true for small spreadsheets, and one reason why I love Excel for quick analyses. Debugging is nearly instant because you see the results as you go along. But it doesn't scale very well.
--If you want to re-use a formula, you basically have to copy-paste it. Generally there isn't much modularity. Programmers are well aware of the dangers of this.
--It's also not a friendly platform for writing regression tests (possible with VB, but who does that?).
--The two-dimensional nature of the spreadsheets and lack of loops means you often need to do hacky stuff to emulate multidimensional arrays or loops. Easy to make mistakes here.
--If you want to re-use a formula, you basically have to copy-paste it. Generally there isn't much modularity. Programmers are well aware of the dangers of this.
--It's also not a friendly platform for writing regression tests (possible with VB, but who does that?).
--The two-dimensional nature of the spreadsheets and lack of loops means you often need to do hacky stuff to emulate multidimensional arrays or loops. Easy to make mistakes here.