I would approach the question from a different direction.
As an empirical matter, there are very few places in the United States were it is even remotely conceivable that this would happen, despite the fact that opposing counsel in legal matters have to cooperate with each other all time and are very good at cooperation and at keeping secrets.
Psychologist Stanley Milgram noted in psychology experiments that even slight steps to make someone identify with a particular role powerfully influences their behavior. People are extremely responsively to even slight whiffs of authority and respond to that role. The Stanford Prison Experiment reached a similar conclusion. People told they are prison guards act like brutes. People told they are prisoners act like victims. This is true even when everyone knows it is just an act and people get very caught up in their roles. This instinct is at work in lawyers too, who, of course, aren't acting.
One can ask why, but the question has to be rooted in the knowledge that not colluding really is what happens not just 99% of the time, but 99.999999999% of the time.
Lawyers as a matter of psychology want to help their clients and identify very strongly with their clients. Yes, they could be punished for colluding with opposing counsel and would probably be disbarred at a minimum and might even go to prison for doing so, but that happens perhaps 100 times less frequently than, for example, simply stealing money held in trust for a client that actually belongs to a client.
Lawyers are much more likely to collude with their clients to illegally take advantage of an opposing party than they are to collude with opposing counsel to the disadvantage of their client for their own benefit.
Now, often lawyers do have to do things that their clients don't like. For example, in a civil case, if a lawyer knows that his client has a damaging document, the lawyer will often have to get the client to hand it over to the other side as part of the disclosure and discovery process in order to play by the rules.
And, often, lawyers will push their clients to accept a settlement because the lawyers believe that it is in their client's best interest, but that their client is an idiot who is too emotional to realize this fact (and this is not gender specific - lawyers have lots of male clients who act like idiots for emotional reasons and have to be talked at length into sanity).
This probably isn't globally universal. I'm sure that there are one or two remote corners of this kind of lawyer corruption in the United States (probably in the rural Deep South, or Appalachia), and I'm sure that there are some countries where this happens somewhere in the world. But, even in some relatively corrupt part of Europe (for example, Italy or Greece), this is pretty much unthinkable these days.