Short answer, because Coda tables are different than spreadsheets. We'd love to tell you more about our philosophy in that link.
If you're thinking about structure in a Coda doc, rows generally represent "things" (like people, tasks, inventory items, etc.) and columns represent "attributes" of those things (like gender, age or address). Having formulas defined at the column level ensures that every row gets the same treatment as the rows above or below it saving you the trouble of remembering to apply formulas as you make changes.
If you need to callout specifics somewhere in your doc, you can always add formulas outside of the table that refer to values or calculations like you would in a cell.