Why are flying saucers rarely visible in a conventional sense? It's so simple: they move faster than light. They don't travel extremely long distances faster than light in such a way that they arrive instantly. They travel through what appears to be many short jumps from our view. The movement between each "frame" is faster than light. This means light does not reflect off of them and they are not visible with our 3D eyes. They can then only be viewed with the 3rd eye or spirit vision.

How did I not realize this before, it's so simple.

A plane or missile moving faster than sound will seemingly appear out of nowhere, you cannot hear it before it hits you. Light waves are also waves like sound, the difference being that they are picked up by the eyes instead of the ears. Anything that moves faster than light is then invisible. It would only be necessary for the saucer to alternate between two very close locations to remain invisible while in place.