Because the charter of projects like LibreOffice are fundamentally broken—they're aiming to replace Microsoft Office by cloning it, but Microsoft Office itself is part of a busted paradigm. It reminds me of that old quote from Linus Torvalds about why distributed version control schemes are worthwhile whereas migrating to Subversion is not—because Subversion aimed to be "CVS done right", and that's a fundamentally flawed premise. You need to make something better that fixes what's wrong with the first thing, and make it obvious/unavoidable what the flaws of the first thing were. To displace Microsoft Office, it isn't enough to try to clone it. (Even if you could, Microsoft Office really only persists due to its own momentum, anyway.)

Microsoft Office is a digital approximation of a paper-based paradigm, but it's not really great at either one of those things. The Web and its plaintext formats are way better for sharing and interoperability than MS Office formats. Some fans of MS Office will dispute this, but they're not being honest with themselves. The fans who aren't lying to themselves will instead point out that it's better for print, but it's only better for print in comparison to the Web. If you're actually serious about print, then TeX and PDF/PostScript are better. But most stuff created in Microsoft office isn't ever really intended for print, anyway. So what gives? Microsoft Office succeeds despite being mediocre on both measures because while the alternatives each have things that make them better in their strong suit, they fail at the things they aren't strong at. Microsoft Office squeaks by because it's able to get a C in all its classes, whereas the others remain well-liked among their friends and family, but they don't get to graduate.