No way to know. Considering there are about 7.5 billion people on earth, you can't know who's doing what and the activities taking place. You depend on media. What if media does not want to cover this? How will you know it then? And it's also possible that aliens if exist.... Why would they even need to visit earth? The universe is 93 billion light years in diameter. Endless number of planets and galaxies. Infinite number of stars. No way to even know what's out there. Earthly affairs is all that's covered

Really hard to say

The definition of alien is an unknown species which there is plenty of in the world. It’s up to people to explore the world we live in to find the real aliens. An alien is not a little green man that you would expect in a sci-fi movie. Creatures and animals that are alien to humans are living in areas of Earth that have not been discovered yet. Some are friendly and others are dangerous. It’s not what you would expect from a sci-fi movie that comes from the imagination of the movie director.

Maybe they have, maybe they haven't got the technology to get here.

I'm going to assume that you mean: extraterrestrial beings with technological achievements at least as great as our own was 100 years ago. *** There are several possible reasons, the most obvious being that it is probably extraordinarily difficult and dangerous to travel from one solar system to another, and if the sort of life described previously is very rare, very much more difficult to travel from one solar system with a technologically advanced civilization to another. *** Realize that it is NOT impossible - for example: we humans could do it with a sufficiently well-equipped multi-generational ship. But the expense of such an enterprise is prohibitive, and the chance of success would probably be pretty small, and (worst of all) the chance of such a ship ending up in a solar system that happened to include a planet habitable by humans is probably EXTREMELY small. *** So: we'd have to be pretty sure that the destination solar system included a planet habitable by humans, and THAT would take, first, a series of probes sent to likely solar systems, which would probably not be able to report their findings until SEVERAL decades later, and THEN (if we found such a planet) we would have to build the tremendously expensive multi-generational ship, which would likely take several additional decades, and THEN train people so that they can handle such a journey (without, say, someone going nuts and deciding to blow up the entire ship)...and then the ship would have to get there without any serious mishaps. Chances are if our probes found a planet with a technologically-capable species already living on it that we would NOT go there, because that species could be a serious threat to our colonists. We would instead choose a planet that is NOT inhabited by beings that are likely to be able to destroy the entire colonizing party...and the tremendous difficulty of interstellar transport is only ONE of the possible reasons why no ETs have contacted Earth yet.

They've probably seen broadcasts from here and decided we're not intelligent enough yet! Either that or they're waiting for us to get along with one another first.