Because aliens either nano-travel, or if they live in the physical universe, they can leave their bodies. The difference between now and in the future is that today aliens come here without permission, trick themselves into entering a human body, and start controlling it. In the future, it will be a mutual exchange.

Of course, this would explain a few things. 

First of all, there are ETs among us, whom we probably pass every now and then on the street, perhaps almost on a daily or weekly basis if we live in a city, without even realizing they are not human.

Barbara Marciniak, in her book, Earth, explains via her channeled Pleiadian entities, that in the future, when humans are multi-dimensional again, and once again will become the Guardians of the Living Library, aliens who match the vibration can come in, ask for permission, and enter a human body to access a certain information they need from the Living Library; it will happen with the human's consent. 

Once the alien soul fragment has received the information they are wanting to access, they leave. 

It can almost be compared to a bee landing in a flower, taking some nectar, and then leave. How can that be? 

Because aliens either nano-travel, or if they live in the physical universe, they can leave their bodies. 

The difference between now and in the future is that today aliens come here without permission, trick themselves into entering a human body, and start controlling it. In the future, it will be a mutual exchange. 

But beware! Don't try this now. If an ET suggests they want to do something like this, don't accept it! If this is the way