They do something about it, it just takes time. Once they remove gilsellers, they come back and restart the process. The only way to deal with it is in bulk. The developers gain nothing from eliminating drone bots, they only succeed if they get rid of the source. When they find the source, they hit hard and take down the command centre. It's like a sting operation or an undercover operation, they have to go in deep and find the core and eliminate it.
What good does it do to arrest a few drug dealers when all the main group will do is recruit more needy people that are desperate for cash? In order to stop it you have to figure out the mastermind behind it and get rid of them and that's what they do with their current botting policy. They study the enemy and strike when they feel they can deal a big blow because if you just keep removing bots all that will serve to do is cause them to play safer and safer, and the safer they try to be the harder it is to catch them so they have to time it just right to eliminate the most RMT in one fell swoop.