Because it's a fundamentally wrong way to do business accounting. It's more difficult to check, more difficult to look for year/year patterns, more difficult to integrate into other tools, etc. A pile of invoices created by vim are a lot more difficult for your accountant to enter in to their systems for year-end taxes than a more common format, like a spreadsheet or a database.
Maybe he does what I do: gives his accountant a categorized list of income & expenses rather than the raw data. My accountant cares that I did $xxxxx in total sales, not that I sold $aa to AA Inc and $bbb to BB Co., etc.
