Separate tools for the register, the warehouse, and the accounts feel free because each one is cheap or already familiar. The real cost shows up later, as the time spent re-entering the same sale three times, and the discrepancies that appear whenever one of those manual re-entries gets skipped or mistyped.
Every hand-off between systems is a place for the story to change: a sale that reduced stock in one app might never have told the accounting app it happened at all. Multiply that by every sale, every day, and "a few minutes of extra typing" becomes a set of books nobody fully trusts.
One system doesn't have to mean one giant, complicated tool. It means one place where a sale is a sale — it reduces the right stock, it appears in the right report, and it doesn't need anyone to tell the other two apps about it.