Can I really not save my spreadsheet as an xlsx file?

I must be losing my mind because there is no way MS hates their users this much.

I'm on Excel online web whatever it's called. I can 'export' as pdf, csv or odf. There is no 'save'. If I try to load odf, it says "filetype isn't supported." (Opened fine in LibreOffice.) There appears to be no way to save a spreadsheet as a spreadsheet that I can load as a spreadsheet.

This can't be true can it? I'm just being dumb, right?