It's all because of that stupid asteroid! Smacks their planet, wipes out the large life forms and leaves the advanced cell biochemistry in place! Terran life gets another hundred million years to evolve!
People like me who evolved on the first try have comparatively simple and easy to digest proteins. We're like a walking buffet tables to them.
Buffet tables with tentacles!
There is no Fermi Paradox! Any time space faring aliens make it to earth, the cows get them!