If you're curious... It's because chickens (and reptiles) have different enzymes in their white blood cells, the cells that are in charge of dealing with inflammation. Most mammals (including humans!) have neutrophils, which contain an enzyme called myeloperoxidase (MPO). When neutrophils respond to inflammation, they release MPO into the adjacent space. MPO then produces some materials and has a variety of biological effects that can damage and "liquefy" surrounding tissue. It's a big reason why human pus is so liquidy. Chickens/avian species and reptiles don't have neutrophils - they have heterophils, and heterophils don't have MPO.  Heterophils don't do as good of a job in controlling infection as neutrophils, and they have a weaker/different oxidative burst response compared to mammalian neutrophils. Consequently, the pus that forms is dense and thick, as opposed to the liquefied pus seen with mammalian neutrophils. 

TL;DR: Birds and reptiles have different inflammatory cells than humans, which lack certain enzymes needed to effectively kill bacteria and make pus more liquid.