We have very limited number of players permitted per map. A bunch of slots are being taken up by PvE characters, often in bot-trains.
The allure is obvious: mobs on WvW maps are super easy compared to regular PvE mobs in lvl 80 areas, yet they drop the same items.
Keep the mobs there, but make them unprofitable for farming. It won’t solve the bot problem, but it’ll get them out of WvW.