Typos are mostly a solved problem in our day and age, but the psychological explanation in this WIRED article is interesting nevertheless:
Generalization is the hallmark of all higher-level brain functions. It’s similar to how our brains build maps of familiar places, compiling the sights, smells, and feel of a route. That mental map frees your brain up to think about other things. […] We can become blind to details because our brain is operating on instinct. By the time you proof read your own work, your brain already knows the destination. This explains why your readers are more likely to pick up on your errors. Even if you are using words and concepts that they are also familiar with, their brains are on this journey for the first time, so they are paying more attention to the details along the way and not anticipating the final destination.