You can track changes within Excel and with Office 2016, there is a comparison and merge tool included (that's been long overdue). But since the files are binary encoded, there is no external way to track changes unless someone wants to write up a parser for the XLSX format that can keep up with all the new features that MS adds every release.
For what it's worth, XLSX files aren't binary. They're just XML in a ZIP. They're not particularly nasty to diff once they're extracted - actually, I think they might do pretty well in Git.
The diff is actually not really straightforward - a sheet is two-dimensional and you have to work out column/row inserts/deletes (which themselves are intertwined) before you can even start looking at cell changes. So it's quite non-standard stuff.
