There's a few reasons for this.

A. They ban in waves, so its hard to notice bannings in the short-term.

B. They purposely let them run around for awhile in the hopes that they can track them to the main account that's dealing with them.

C. There are a lot of them, and they have to personally check to make sure they are bots, they can't just ban all of them based on a few reports. When they happens its a headache for everyone involved.