We have you guys. We really do depend on your flags, and not just for this. The last time this happened - which was very recently - flaggers alerted us to the fact that this was an extended issue. Your flags make a huge impact.
Moderators depend on user to watch everything. There's no reasonable way three people can (or should be expected to) read every post and watch every message. It's our job to watch stuff too, and we often do find things on our own, but nearly everything we act on was flagged by a user. It's everyone's community, and it's actually regular users that have the majority of the powerful tools of moderation.
Once we're aware of an issue, moderators have tools designed specifically to find and handle questionable accounts. For obvious reasons, I can't go into details on how these tools work and what they specifically do, but suffice to say they tend to be quite good at their job. Anyone trying to pull this will likely be shut down very quickly.
This combination means it would be awfully difficult to go un-noticed doing this. Frankly, even two posts linking out to the same site on any user's account would be enough to prompt me to probe further - not necessarily take action, but look and see what's up. I do the same thing when I see a new user whose first post links out to an external source.
However, the last line of your question raises a good point, and it's something I've been thinking about as well. My go-to answer is that we should cross this bridge when we come to it; that might be sooner than we think, or it may never become a serious enough problem. If it becomes a problem, we can do something about it reactively.
Still, a close reason specifically for challenge questions that are copied from elsewhere might be warranted right now anyway. It really depends on how you all feel about it - whether these questions are a benefit to us, and if they're not, are they detrimental? They can be a pain to deal with, but to be fair, that's almost definitionally everything mods have to handle anyway ;). This is a discussion for another post, though.
