In politics for every side there is an opposing side trying to find a new way looking for an out, and if that project appears plausible, cost a lot of money and employ a lot of people then it will even more likely make it even if it's otherwise a bad idea, .. but it still happens, because something has to get done, people expect change and things to get better and cannot predict exponential runaway growth.
Also when the wall project was started with class 1-2 Kaiju every couple of months, the wall as envisioned might have been good enough to stop them at least entering cities, and much cheaper over the life than constantly maintaining $1B jaegers, they didn't expect 2 and 3 at a time in 2-3x the size.
I'd also imagine that such a wall would have armament like cannons on par or higher than what could be carried by many yaeger which along with aerial megasize bombing is one of the biggest holes in the movie as far as I can tell to stop creatures...especially when striker eurka kills one with chest mounted missiles, gypsy kills 2+ with the plasma cannon, and others use close range blunt force or slashing attacks, cause if there were 1B spent on a high energy or other projectile weapon there wouldn't be much of a human centric plot. As the rift is single known, just build the weapon near that and anytime one pops out, pop it in the head, or nuke around the entrance to block off the entry point.
On that note, things aren't much better in the real world. We humans do lame expensive things all the time, like perpetually build mobile home trailer parks in the middle of tornado ally, super expensive not completely-earthquake in earthquake prone zones, and whole cities beneath sea level (e.g. New Orleans).