Because usually bullet sponges are boss fights, repeating same pattern and shoot certain spot (which not know, until you find it. Or sometimes you need the crystal ball to discover it).
Second thing, 2 story house size of machine thing, would probably be pretty much immune to all sorts of bullets. Because size, and massive amount of metal would make natural armor.
Third thing, this size of machine would have so massive fire power, that single player would not just simply have any sort of chance against it. It would just obliterate everything where player is.
Of course all of this is highly hypothetical if there was even small part realism in this game.
Bullet sponges are just boring tactic to make game last longer. And these type of games you just gun blaze trough the ordinary mobs, and then come to this boss fight.
How this game goes, keeping you on the edge. And any fight can lead to your death if not careful. That is the true excitement in games.