@aesthr

I understand why, and if you don’t want to be traumatised, I suggest you stop reading now.

A spreadsheet is not actually the document unit for Excel. Any open sheet can refer to cells in any other open sheet (hmm, I wonder if anyone has exploited that with sheets that fetch from external data sources to extract other information). By default, any cell reference is in the current sheet, but you can include other files by name and their cells will be used but only if they are open. This means that any editing operation may affect any other open sheets and so, logically, it follows that undo should apply to the document (all open sheets) and not simply one sheet.

This is also why Excel does not let you open two sheets with the same file name at the same time. If you do, cross-sheet references would be ambiguous. Remember, they don’t refer to files as paths or some kind of unique document identifier, they reference other sheets by file name from the set of open files.