Taken to it’s logical end, a country without borders is NOT a country. A geographical entity has no need for government and laws. Therefore, not building a wall deprives the Democrats of that which they crave most - power.
Are they prepared to grab power from the barrel of a gun? I don’t think many would survive to enjoy that power.
I would contend that some, perhaps most, understand the facts but the facts are irrelevant to their Project. While perhaps they understand the facts it is to their advantage that the voters do not. Social Democrat control of the schools from K to PostGrad and of the Big Media(including e-media) and of Circuses(pro sports and all aspects of Entertainment)facilitate that.