Security. Security cannot realistically be automated or scaled. Because any automated system at scale can be understood and gamed. And because humans interact w

Security. Security cannot realistically be automated or scaled. Because any automated system at scale can be understood and gamed. And because humans interact with secure systems, and humans can be tricked. Even when you built a system that's say, cryptographically secure, you manipulate the humans to work around it, because humans aren't good at identifying cryptographic keys, and all automated systems serve human clients or masters. This is why the Play Store and Chrome Web Store are full of malware, and why human-curated app markets have effectively zero.