For many reasons
1. Some of the time these people do this in areas where specific mobs need to be killed to accomplish tasks and their persistant nearly afk farming means no mobs to kill.
2. For some players it completely removes the immersion of the game world. The unsightly cluster of players nearly afk or afk farming totally breaks with the narrative/feel of the games world.
3. Some players view it as lazy/scummy behavior that is to be discouraged because youre not actually playing the game just abusing mechanics, or loose TOS guidelines.
4. This type of behavior negatively affects the market on certain items making them super cheap when they shouldnt be, and gives players who arent actually actively contributing to the games community a decent income for what many consider slumming.
5. A portion of semi afkers and afkers are RMT traders who are running as many accounts as they can to earn money efficiently with very little input so they can then sell the Gold online to other players. This type of player is 100% against the TOS, and also by most gaming community standards absolute scum. So when players see someone 'botting', afking, semi afking, they assume they are RMT traders and that they need to be banned permanently.