Stephen Webb is an author specializing the Fermi Paradox and other cosmological subjects. I have previous linked to his book 50 solutions to the Fermi Paradox.
A new edition of his book is available and he has added 25 more possible answers to the Fermi Paradox. If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life
I still prefer Occam's Razor. The reason we don't see any intelligent aliens is that there aren't any.