CCP created plex specifically to reduce the impact of RMT. it hasn’t been 100% successful, but they did it LONG before any other companies tried anything similar.
They also do ban people caught RMTing on a regular basis, they might not catch everyone but they do catch a lot of them. CCP also tends to ban in waves, they will spend a few months gathering names and investigating to make sure that it is RMT, and then do mass banning. They do this to slow down the rate that RMTers change tactics and it allows them to gather larger batches of IP’s.
The police don’t go out and bust every single small time drug dealer, they wait until they have enough information to take down entire rings.