A CSV file may use a different field separator from the one Excel expects under the current regional settings. When those separators do not match, the data can open in a single column instead of separate fields. The extension alone does not guarantee that the file uses commas.

Diagnose CSV Data That Opens in One Excel Column

The Ablebits guide discusses semicolons, tabs, quotation conventions, and encoding differences as sources of conversion trouble. Inspect the actual delimiter before changing settings or rewriting data. An import workflow with deliberate separator selection can resolve the issue without treating the whole text line as one field.