Many reasons. First off, an opaque wall blocking all vision to the other side is not a good thing, what makes the current fencing good is that border patrol can at least see when someone is trying to get through (they literally said this themselves). Most illegal immigrants at this point didn't cross the border illegally, they crossed legally and overstayed their visas. The ones that the wall is trying to keep out (organized criminals) are the ones that will be doing things like building tunnels or finding alternate routes, the wall would mainly keep out normal people that are just looking for work to support their families. That right there is the biggest reason the wall won't do much, but that's not even why it shouldn't exist. $5 billion is barely a down payment on this thing, the real cost is more like $25-$70 billion, and that's just for materials and construction, not to mention all the legal battles that will ensue over property rights, protection of natural resources and landmarks, etc. Then you have to massively increase funding for the border patrol (which we already just did) and you're looking at $100 billion+ in total cost. Then you have to maintain the wall and repair it, and so it goes on and on. Basically, it's an exceedingly expensive measure with questionable morality that doesn't even fulfill its true purpose, keeping criminals out of the US.