Congestion is the most likely culprit.

They can't replace the bots that get routinely banned in waves.
And, they can't world visit to the Congested servers to spam there.



It's one of the reasons I preferred to be on Balmung back in the day, before RMT was taken seriously and dealt with.
The server being constantly too full for them to make spam bots made it so tells were actually a useable feature on the server.