What is DOCX?
What Word has saved since 2007: a zip of XML parts, replacing the binary .doc that came before it. It is a page format through and through - paragraphs, styles, revision marks - with no notion of a typed value anywhere in it, so a table in a Word file is text arranged in a grid rather than data.
What is XLSX?
A modern Excel workbook, which is really a zip full of XML. Unlike a CSV it knows what a number is, holds multiple sheets, and keeps formulas and formatting. It's what people mean when they say 'send me a spreadsheet'.