The revenue is mostly from selling in-game money and rare items which are farmed by bots. There are many trading websites you can use, just search "buy WoW gold" for instance. (I just looked and this market looks incredibly saturated compared 15 years ago).

Selling the bots themselves was an option, but people can just reverse engineer your bot and redistribute it. The bot typically ends up posted on some public forum and becomes a target for the developers to patch against. Also, the more people who are running the bots in game, the less profit you are going to have from selling your items because they're more abundant, so it is better to keep it confidential and just have many accounts farming.

Before the bots were commonplace, there were Chinese "sweatshops" where people were paid scraps to play the games for 20+ hours a day in horrific working conditions. Would hear stories like "man dies from playing MMORPG for 36 hours straight".