Why owners stay in spreadsheets
Spreadsheets are free, familiar and flexible. An owner who built a tab per month and a summary sheet feels in control. Accounting software looks like a cost and a learning curve, and accountants have sometimes made it worse by insisting on complicated setups.
Respect that. The spreadsheet got the business this far. Your pitch is not that the owner did it wrong but that the business has outgrown it, which is usually true once it is VAT registered or facing a corporate tax return.