Some theories why we haven't found aliens.......

[Justdroppedin]

Other theories.....

Intelligent species might be inherently competetive, as humans are. As such, wars are inevitable. They might have wiped themselves out, or at least knocked their progress back repeatedly, by nuclear war. We almost have several times; it's not implausible that our near-misses are the exception, not the rule.
Similarly, they might have industrialized so heavily that the planet becomes uninhabitable before they can escape the consequences. (This may be what they mean by the "Gaian Bottleneck", but I found the explanation of the term unclear so I thought I'd mention it.)
There's the Reservation theory, in which Earth is in a protected region of space akin to a nature reserve, where aliens aren't allowed to go. There's multiple possible reasons such a policy might exist; maybe we're in the middle of a border area between two empires, or we might be considered "wildlife" who must develop on our own, without interference, similar to Star Trek's "Prime Directive".
There's the Ascendant theory, in which alien life existed but has moved on. What they've moved on to is anyone's guess, but they are no longer concerned with the everyday world we know. A variant on the Great Silence.

[The Noof]

It could be that Extra terrestrials have also been searching for intelligent life outside of their own existence, and have already discovered that there is none here

[Justdroppedin]

It could be that Extra terrestrials have also been searching for intelligent life outside of their own existence, and have already discovered that there is none here

I would agree......probably the most valid reason.....

[steveOtoo]

I think the Aliens lock their doors when they drive by...................

[kenkell1]

It could be that Extra terrestrials have also been searching for intelligent life outside of their own existence, and have already discovered that there is none here

Exactly LOL......