Why isn't this preached?

I think the reason is it is embarrassing. There you are ministering, speaking about it, and people respond with a "duh," that is so obvious. You feel like an idiot that you have to tell a congregation of believers this is what God wants. But they look back at you dumbfounded nonetheless because they are no less guilty than you are for not doing anything about it. In other words, they sit there in the pews in a particular qualified denomination, or non-denomination ("I of Christ"), and don't make Scriptural locality a reality by walking out of that building and fellowshipping together under the name of the church of the city they are residing in.

I think the denominations have made it too easy for people to not think for themselves, walk into their premises and passively listen to whatever rationale is put forward why their denom is the right one. No denom or non-denom is the right one. That is a fact.