While it’s super annoying for the tech savvy, and gives a great opportunity to ill willed tech people, I’m sure it was an idiot proofing move. The average user is a not-so-tech-savvy office person, having relatively fuck all knowledge on extensions, and back in the day pretty much all programs got picky when facing an unknown/unsupported extension. Your average Joe/Jolene opened ‘veryimportantspreadsheet.xls’, renamed it to ‘veryimportantspreadsheetnew’ (without the extension), and made it impossible for Excel to open it by double clicking. Then in the best case they triggered an IT support request; in the worst case they reported that the very important spreadsheet got lost/corrupted and data was lost.