In a word - speed. An index for up to a million rows fits in a 32-bit word, so it can be used efficiently on 32-bit processors. Function arguments that fit in a CPU register are extremely efficient, while ones that are larger require accessing memory on each function call, a far slower operation. Updating a spreadsheet can be an intensive operation involving many cell references, so speed is important. Besides, the Excel team expects that anyone dealing with more than a million rows will be using a database rather than a spreadsheet.