The matter of seeing, and how to see, is a very serious academic issue.
Simply put, the world is full of information, but the way to receive information varies among different races.
If we compare the eyes to a radio, then information is like frequencies. For humans, perhaps from the moment they are born, their eyes receive the 92.7 traffic broadcast channel. So naturally, what they see is the information within this frequency. But for ghosts, perhaps they receive the 105.2 music broadcast frequency, so they cannot receive the traffic broadcast frequency. Therefore, humans and ghosts cannot see each other, let alone communicate with each other.
Of course, a very small number of people can switch the frequency their eyes receive between 92.7 and 105.2, which is known as the Yin Yang Eye. When you tune to the former, you receive the traffic broadcast like everyone else. However, when you tune to the latter, you receive the music broadcast.
