Aliens never came to earth because they don't have the technology. Their technology is the same as ours. For aliens to come to earth, they'd need a spaceship. To build a spaceship that can come to earth, they would either have an impossibly big brain or have existed an impossibly long time.
Also, if aliens came to earth they would probably not have used a saucer shape as we imagine alien spaceships. They would use a more aerodynamic shape. Now you know one of the many opinions of why aliens never came to earth.
