I think it's because people have a hard time seeing things from more than one perspective. We live our days and react to things based on our perception. We thi

I think it's because people have a hard time seeing things from more than one perspective. We live our days and react to things based on our perception. We think about our state. Few people think about how things look from another person's perspective. Many people have a hard time really getting into someone else's shoes. With multithreading, people can reason about what one thread will do. They can also reason about what another thread will do. However, thinking about the various unexpected ways these threads could interact, and designing a protocol for those interactions to limit adverse behavior is something many people are bad at. One perspective at a time is OK. Holding 2 or more at once is tricky.