Why would aliens bother to exert themselves? Just wait 50 years and we'll kill ourselves without any aliens necessary.

I no longer believe that intelligent, space-faring aliens actually exist. I now believe that any creature intelligent enough to travel in space, and aggressive enough to want to expand across the galaxy, will destroy themselves in the same way we are doing right now.

I didn't invent this idea of course - it's called The Great Filter: https://en.wikipedia.org/wiki/Great_Filter