A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
When the data is in a single column the CSV file will have what you indicate - that is how Excel always worked
If you want abc,def,ghi,jkl,mno then the data must be in a single row
The first screenshot shows Sheet1 with the original data in column A
Using Copy and Paste Special I transposed the data to C1:G1
Then I deleted columns A and B
Then I did as SaveAs specifying CSV
The second screenshot shows the opened in Notepad
best wishes