Because that would take work.
But in all seriousness, I think in part that some of the banned members were either actually a large portion of the population here that could engage semi-productively (when prompted correctly) or were at least catalysts for intelligent discussion - whether or not they would in turn respond to the discussion intelligently is another question...
The only one I can recall that I found useful was Base Groove. The others were far more disruptive than helpful, from my perspective. *shrug* That's just my personal opinion.
I for one advocate for what you've proposed. But it seems, even despite the support these types of pre-solutions have garnered ITT and others, that nobody is really willing to take the next step and engage people in some type of active discussion. In other words, we have the forum, and we have people willing and in fact desiring something similar to what you have said. But we have no way to ignite this fuel - my suggestion was to consider some sort of ignition; whether it would be ultimately destructive or not would have to be monitored.
I think one issue is that people come to forums for different reasons, and/or invest their time in varied ways. So it's not really a uniform type of posting process.
For example, during part of my life when I had the time and was looking for a particular type of connection, I invested far more in my posting. Nowadays, I don't feel the need for long dialogue or expanding certain types of understanding for myself because my needs changed. I enjoy reading some threads (for self-education), but my posting is more casual than it has been... I look more enjoyment versus an intense investment. And I provide advice if it looks like something I could offer would be helpful to someone.
Likewise each individual has separate needs and these needs could change over time. So one person's vision for the forum can differ from another's; and even the same person's vision could change over time.
Anyway, styles of interaction that discourage others from participation I think are unhealthy. I'm not talking about one's level of background in the topic -- you can always read and learn, or ask questions -- but if you're going to interact in ways that either make the conversation habitually tedious or abusive of other participants... not good. Those types of members, I'm not sorry to see go.
I don't think many people here are social enough to know people to invite who would fit. we kind of each just wander in and stay. In fact, this is where some of us do the "meeting" of others.