“Why haven’t we been visited by an infinite number of travelers from all the universes that have mastered this?”
A good question. “Life forms” totally alien to us may not show themselves in a manner we as humans are capable of sensing. With this being said, it is quite likely we have been visited by creatures from another dimension/universe.
It is also likely animals with certain senses that are either superior to human or that have senses we are not aware of, have actually encountered such travelers.
It is also likely as humans with limited senses and technology, that there is a parallel dimension and we are simply not aware of it. While we are not aware of it, creatures in that dimension could very likely be aware of us all the time. Just because such creatures may exist, does not mean they have to make themselves presentable to us in a manner we can detect.
Maybe your alien life forms are what is taking thousands people all over the World. Search & Rescue people can’t find them and some say that it like “they disappeared off the face of the Earth.”
See the series of Missing 411 books by David Paulides:
http://www.canammissing.com/missing_411.html