Plenty of carriers start on a spreadsheet and it is a reasonable choice at the very beginning. The costs are hidden rather than absent, though, and they grow with the operation.
The first thing to break is usually billing accuracy. A spreadsheet does not enforce that every accessorial on the rate confirmation makes it onto the invoice, or that the right documents are attached before it goes out. Small leaks — an unbilled stop-off here, a lost lumper receipt there — add up.
The second is shared visibility. Once more than one person touches the file, or once you need to answer "what did we net on the Atlanta lane last quarter," a spreadsheet either becomes a fragile web of tabs and formulas or stops giving trustworthy answers.
The third is documents. Proof of delivery, rate confirmations, and weight tickets scattered across email and a folder on one laptop become a real liability the first time a customer disputes an invoice or a claim comes in.
A spreadsheet is a fine place to start and a bad place to scale. The move off it is usually driven by a specific pain — cash flow, a dispute, a second truck — rather than a calendar date.
Terms in this answer: Proof of Delivery (POD), Rate Confirmation