The simple answer to your question is: Because many of these exploits don't get reported with enough details for us to get them fixed.
If you are annoyed that other people break your game experience and you know how they do it, then say something in the
Report an Exploit part of the forum instead of just assuming someone else has already reported it.
Reality is that few people actually report these things.
For me to talk to people and go like "Hi, I'm Genele. I think you are exploiting, would you mind telling me how you do it?" How many people do you think would actually tell me anything?
We have to rely on honest players to give us information, that is just how it goes. It is a lot easier for honest players to be able to infiltrate exploit communities to get information than it is for us developers. (True story, my play characters' organization got so paranoid that all their exploits got fixed that they stopped talking about them in the org channel...)
Also, the market exploit that everyone seem to talk about was fixed almost a year ago.
(I got called back from vacation at the time so I remember it clearly...) The reason why we found it that time was not because someone reported it, it was because one of the people who exploited went so far that the market system crashed. The biggest issue with cleaning up after that exploit was the fact that people who didn't intend to exploit were used passively by the exploiters. We did however go through all the market logs and all the money and items that had been exploited in the market system at the time was removed, many players were banned and anyone claiming something else is lying (After all I went through the logs with CS myself during my vacation, so its a bit hard to forget.)
If you know of any other issue that has not been fixed, again please let us know how it is done so we can fix it. We always prioritize fixing exploits when we have enough information to get it done in a timely manner.
At the end here I just want to say that we can't just stop developing the game because a small group of our players have decided to make it their life's purpose to break the game for everyone else. You also have no idea how much time we actually spend on fixing exploits. Every time I write in the patch notes that we have fixed exploits, its because we have.