Wait, it's the top-selling book, so a lot of people don't have it? What?
Scribe was asking who doesn't own the core books. The core books are the PHB, MM, and DMG.
There's enough content in just the PHB to play for the rest of your life, and the MM and DMG just add to that. Nobody actually needs all the supplements. Plus, there's multiple subreddits and many, many blogs dedicated to fanbrew archetypes, classes, races, monsters, spells, magic items, and even entire adventures. Heck, I once posted a document of 200+ spells converted from 2e to 5e to r/dndnext; I still get people DMing me about it. I created a version of that for Level Up, and have been posting monster conversions to the Level Up forum here.
If you want extra content, it's out there, for free, and some of it is better (or at least more interesting) than stuff WotC produces.