Good question. I wish I had a better answer than what I'm about to say.

It just is.

I'm a diehard anti-Windows, Linux-lovin', FOSS crusader myself, but if Microsoft released a copy of MS Office for Linux (as a one-time-purchase), I would buy it today.

For most tasks, you're right, there's not much you can't do in LibreOffice. But the interface is clunkier. Excel makes it easier to make good-looking spreadsheets. And as much as it hurts me to say, looks matter when dealing with nontechnical folks.

Plus there are some things that are just more intuitive in Excel, like certain kinds of charts and graphs. There are some advanced features of Excel that don't even exist in LibreOffice. Like chart styles and certain team collaboration features.

Compatibility is... okay... For the most part, but having it all guaranteed by a bunch of paid devs would be really nice.

There is a more detailed list here

I can't speak to the privacy of it, but I love my Kobo Elipsa 2E setup. I've got NickelMenu, Synching and Autoshelf installed, so I can sort books by directory and have them sync. I also have KOReader installed with some other goodies, but rarely use it. The reading experience in the stock firmware is way nicer. And the screen is big enough to read full size PDFs comfortably. Kobo runs Linux under the hood, and there are extensions to give you Telnet/SSH access so you can mod what you need.

Also, being able to borrow books from Overdrive/Libby on it and support my local library is rad.

I did the whole Calibre with Kobo extensions thing for a few weeks, but it just didn't work out very well for me. I like to have my books synced to multiple devices and Calibre just isn't built for that.