Since we have never been visited by aliens, or witnessed time travel, these things must not be possible. It they were, there would be proof and it would be documented. It's as simple as that.
Hand-wavey sci-fi explanation - time travel being observed causes a divergent reality (think many worlds), we're in the branch that didn't notice. Seems like not much more of a leap than aliens/time travellers would definitely visit us _and_ we would notice.
> Since we have never been visited by aliens
This is a rather large assertion to make.
So is the "if it's possible, they certainly would have done it to visit us" leap of (non)logic.
Eh, I just feel that there's more "evidence" of ET's being seen than not - albeit just through religious texts, historical paintings, eyewitness accounts etc. Plus it's more entertaining to consider it as a possibility than to be a wet blanket and write it off completely.
If the greys figured out how to achieve FTL, then they probably also have the means to be covert as well - assuming passive observation is their goal in the first place.