MS-OOXML is barely an open format. Have you tried implementing it? ECMA-376 part 1 is over 5000 pages, and there are four parts to it! (Part 1 contains an extra bit about SpreadsheetML, but by that point we were two zips deep. I turned back ere I got lost.)
The OpenDocument formats, meanwhile, are older, simpler and better than their MS-OOXML equivalents. (The ODF spec is 1041 pages altogether – 215 pages of that are the spreadsheet formula language.) LibreOffice's implementation is a little janky, sure, but I can edit OpenDocument files by hand. Try doing that to a MS-OOXML file. (Good luck.)
And yes, Excel still fully supports .xls too.
I fear whatever format LibreOffice uses will die first, case in point I don't even remember what it's called even though I should as a computer nerd.