This study explains my favourite solution yet.

The paper I hyperlinked explores the possibility that low mass k-type stars are similar to our own g-type except are more stable and can outlive g-types 3-6 fold. It also explores k-type distribution density in the galaxy and concludes they are more highly concentrated and much closer together in the centre of the galaxy. Given these facts they propose that there is a migration pressure for space faring galaxies to move in towards the centre of the galaxy, and if there is a “Galactic Club” of aliens chilling out somewhere doing awesome shit, then they would have most likely just colonised the centre galaxy k-types and left all the other solar systems alone. Hence we are sitting on an outer solar system g-type that is not worth the time or energy to colonise, hence we appear alone.