Only CL Staff know the "true" answer.< read-learn-succeed >
The best anyone can do is offer speculation and conjecture.
One possibility is the most avaricious one: CL Staff like money. Dealers paying to post thousands of "bad" ads bring in considerable lucre. Power may corrupt, but steady revenue is in a close second place.
Another less corrupt possibility: It is the readers themselves who are not paying attention. Sure, you are flagging all of these advertisers, but how many other readers are? You cannot know with certainty how many flags any given ad has, nor how close it comes to staff review.
CL Staff have said in the past, they will solve "problems" when they become actual problems — i.e. when it starts to impact metrics that are important to them.