One hypothesis for why we have not detected alien life is that as a civilisation technologically matures, it stops physically expanding and begins to develop the ability to live in virtual worlds.
The argument is that there is no need to continually physically develop when you have the ability to simulate life. Why bother physically building something that you can instantly virtually replicate?
Basically after our technological adolescent and expansionist explorations, we turn from outer space to inner space... our journey undergoes S.T.E.M. compression, the compression of Space, Time, Energy and Matter- until non biological minds live inside virtual worlds at the nano and femto scale, further compressing complexity until we create black hole-like conditions and disappear from the visible universe altogether. source
If the transcension hypothesis is correct this would explain why we cannot detect any trace of alien life.
If all universal intelligence eventually transcends to black-hole-like environments, after which some form of merger and selection occurs, and if two-way messaging (a send-receive cycle) is severely limited by the great distances between neighboring and rapidly transcending civilizations, then communication with feedback may be very rare, an event restricted to nearest-neighbor stars for a very brief period prior to transcension. source