Because it's the best spreadsheet software out there. It really is first-class, and LO Calc has a lot of catching up to do (and they are quite aware of this).
My theory is that Excel's userbase includes the guys who actually sign the cheques, so MS has to get it right.
Yeah... LO may have 80% of the features, but if 25% of users use a few of those other 20% of features, you're going to face a lot of resistance from any company with even a dozen users. I've seen some amazing/scary things done in Excel (and Access even).
I also like Access for what it is. I haven't used Base enough to really comment... Would be kind of cool to see Base spun off, so that it could be a stand-alone access competitor, if that isn't already an option.
There are also many computers, where nothing but Windows, Office and company ERP client are installed. This has been for decades.
Due to lack of other tools, people learned to do absolutely crazy things with the Office - from saving screenshots in the Word files to drawing in PowerPoint.
My theory is that Excel's userbase includes the guys who actually sign the cheques, so MS has to get it right.