"On the sci-fi show "Lexx," the planet Earth was classified as belonging to a particular class of technical planet which inevitably tried to used massive colliders to determine the mass of the Higgs boson and ended up collapsing the planet to the size of a pea. That this happened often enough to require a specific classification for this was funny, but it makes one wonder exactly why there are no advanced civilizations being detected out there... "

I would think that it would be a rather rare event in which a civilization wipes itself out because of some "mad scientist". Nor even because they fought some war and some super weapon did them in. On Earth, civilizations have failed, and their ending usually related to poor leadership- bad management & unsound economics (read: motivation for individual citizens to "contribute to society"). I would say that civilization generally end due to the "everyday decisions" rather than some unique event- though I think it's probably quite common for some single event to trigger a collapse of a civilization- sort of the "last straw".

I think the reason some alien hasn't knocked on our door is due mainly to the vastness of space. An analogy would be planet Earth which instead of merely being large, would be "practically" infinite in size. Instead of a jungle being confined to a relatively small area, think of a jungle which so large it has more surface area than Earth- and that's just one jungle of many, then there are large deserts, ect. In this large imaginary world, large towns may never or at least take a much longer time to develop: social groups remain small and mobile. Instead of humans exploiting all the Earth's ecologies, they keep to a few of the easier ones. Add in diseases and other factors, and you get a stagnant population.
For space cultures, the major stagnanting element is the distances and time.

As far as detecting aliens, I think the simple answer is we don't have telescopes with enough resolution, and if we did THEN we would have the problem finding them in the haystack. With the current radio telescopes we could detect an alien signal, if they are broadcasting a radio signal at us, at the right time and with enough signal power. We are actually in a pretty "passive" position- dependent on an alien culture or one alien wanting to contact us.