It used to be that mortals were well aware of the mythological world around them. Even the Emperors of Rome had been well-known as demigods. As faith waned in the Greek and Roman gods, though, so did their ability to see through the Mist. Now, it's a rare gift for mortals to have that kind of sight. Some demigods even have a talent for manipulating the Mist, and Hecate's children are definitely proficient enough with it to use magic.