If all you want is an honest explanation, Praxas, then I shall explain. Guarded physical barriers are generally the most effective means at preventing illegal entry into a country. Where they are put in place (and actually guarded) they tend to work quite well at preventing illegal entry. In the United States, where we have guarded barrier walls and fences, they tend to work quite well too. Physical barriers allow a nation to guard a larger stretches of land with fewer men and women needed to guard every foot along the border, acting as a force multiplier, because if people want to get through, they have to concentrate their efforts (getting several people together to help climb over at a specific point, or dig under). As a result, illegal entrants can be interdicted more easily and effectively at the points where they attempt to break through, whereas they can more easily get around unguarded parts of the border or scatter when they see guards.
That is my honest explanation, but as you say, you do not appear to care whether it is the best solution or an effective solution, because even if you were to be convinced that a barrier were effective, giving Donald Trump the funding for it would be giving someone you find morally reprehensible a political victory. So I may as well be urinating into the breeze, but I guess I am a glutton for punishment.