Why are my excel-files considered corrupt if they are only unzipped and then zipped again?

I am writing a C# application which modifies excel-files with a third-party library (ClosedXML), but upon saving the files with the library Excel recognizes them as corrupt. I can't open them either by code or by Excel itself. I even asked the maintainer of the library about it but for him/her there was no error.

This is the scenario I have:

1. Create an empty workbook, save it as an .xlsx file
2. Rename the file to .zip
3. Unpack the file to a folder with 7zip
4. Repack the folder to a .zip file with 7zip
5. Rename the file to .xlsx
6. Excel can't open it because it's corrupt.

I changed nothing about the contents of the folder(s) or any of the xml files within the disassembled xlsx file, so how can it be considered corrupt?

Is there a setting in Excel to recognize some xlsx files as corrupt? Are some file attributes missing? What am I missing?