In animal farm why does he describe the animal as slaves in chapter 6

During the following year, the animals work harder than ever before. Building the windmill is a laborious business, andBoxer proves himself a model of physical strength and dedication.Napoleon announces that Animal Farm will begin trading with neighboring farms and hires Mr. Whymper, a solicitor, to act as his agent. Other humans meet in pubs and discuss their theories that the windmill will collapse and that Animal Farm will go bankrupt. Jones gives up his attempts at retaking his farm and moves to another part of the county. The pigs move into the farmhouse and begin sleeping in beds, which Squealerexcuses on the grounds that the pigs need their rest after the daily strain of running the farm.