- It's uncomfortable. You are typically so excited about an idea that the thought of learning it doesn't have mass appeal is uncomfortable. People avoid uncomfortable realities. In other words: "I KNOW this is a great idea, I just KNOW it. If I don't build this NOW, someone else will beat me to it. I don't have time to waste talking to customers. If Henry Ford would've asked people what they wanted, they would've said a faster horse."
- People are proud, if they're going to show a product to their friends/family, they want to make sure it's well-built, fully featured, etc... The idea of "just getting something in front of customers that works" aka an MVP is embarrassing for some people. They think that if they release a "amateur" product, people will think they're amateur.
- People are irrational. They think that their idea is so special that asking people about it, getting validation, etc... could make them a target for idea theft. So they isolate themselves, validate with a few close friends (who are unlikely to give honest, brutal feedback) and build a huge product so that when they launch, they'll be far ahead of their competitors. The problem with this is that you'll likely build the wrong product the first time around.