This is a fault of the AI, but Warband's multiplayer places intelligent minds behind those masses of bodies. Smart players who flank the castle walls, who breach the parapets and whose fights spill over into courtyards and throne rooms to claim the castle. Multiplayer Siege plays like the single-player fantasy.
You are also conviently forgetting the other half of the players in the same servers who consist of
-Racists/alt right wingers trying to start fights on the text chat
-Mic squeaks and the guy vaping playing his music in his mic, while a vacuum cleaner is roaring in the background.
-Trolls who will try and throw the game, intentionally hitting teammates, directing siege weapons to aim at teammates, running over teammates with horses, interrupting archers wih kicks
-People who just simply don't care about winning the game, playing joke loadouts, running to get themselves killed, or hanging out with their friends at spawn point to do a funny visual glitch.
-Hackers and scammers