Why no save as xlsb?

Spreadsheets can be read from xlsb, but not rewritten. If we know how to read one, we should know how to write one.

For reference, an XLSB file is apparently an “Excel Binary Workbook File” and supports the same feature set as XLSX, but the binary-encoded file can be saved/opened much more quickly. more here

not quite. It’s lots of hacking, as there is no open documentation. You may have known an epopee with NTFS support in Linux. In short: they learned to read it quite long ago, but writing corrupted filesystem till NTFS-3G in ~2007. Same with xlsb: they can write it, but they must write it so Excel would read it back, which is way harder and requires lots of trial and error.

Feel free to submit a feature request for XLSB export. Is there a specific task in which you need this export functionality?

As XLSX and XLSB both seem to support the same featureset, would it be possible for you to save changes in the XLSX format?