If something gets past the planetary defense cannon, what is stopping the people with nuke-tipped rockets blowing them out of the fucking skies? They would do it for a fraction of the cost, just as well or even better than the cannons.
I guess their people like being bled dry by oppressively high taxes that are wasted on extravagance. You sound like Mitt Romney saying we need to tax the people to make more battleships. "Lets make something worse than the much cheaper more elegant versions for no reason other than to wave our dicks around"
As a response to your edits: jesus christ, I didn't mean a car literally! I meant some sort of rugged artillery carrier that is protected by infantry with guns to shoot hostile wildlife that would be all rights be scared shitless by the deafening sound of heavy artillery.
Also, bigger round does not necessarily mean "necessary to kill big things". Why you can't have a fast round that penetrates and then explodes. You don't need a large cartridge when you have mushrooming lead or just kaboom, but even if you do want something big FUCKING EIGHTY MILLIMETERS IS REDICULOUS! The biggest round the US uses in a machinegun is the 30x173 on the GAU-8, and that is meant to kill T-90s, which I fucking bet are better armored than some wild animal. Also, dear god in heaven how long will the round be? The 30x172 is already a really long round by necessity, so imagine an 80mm one! Motherfucker would be half a meter long!