The same disgusting concept seems to be why the Pattersons are never grateful for anything. Since they aren't thanked enough for what they do, they don't see the point in celebrating gratitude. The problem with that logic that isn't it being self-serving and repellent is what the Pattersons need to be thanked for. For starters, it's quite clear that Elly is still to this day pissed off at her children for not thanking her for changing their diapers, feeding them and so on and so forth when they were, oh, two or three years old. You'd have to be a vindictive idiot to get that upset but, hey, Elly is both things.
The other thing that Elly and John expect thanks for are things that they shouldn't actually be doing. Elly shouldn't actually be making work for herself and John shouldn't actually be doling out stupid, harmful advice that just makes everything worse.
The end result is that since they aren't thanked for necessities and idiocy, they have no reason to be grateful for anything.