Why do villains try to fight the Avatar?

Amon/Noatak said it best: "You always say bloodbending is the most powerful thing in the world. But it isn't. The Avatar is." Every time a new threat appears, the Avatar always finds a way to beat it. A tyrannical nation that's been under control for 100 years? Just give the Avatar a few months. Lost your bending to a bloodbender? The Avatar now has energybending. Dark spirits in the mortal and spirit world? The Avatar only needs their bare hands. Not to add to the fact that they always have a Team Avatar made of competent benders and non-benders. So, what's an in-universe reason for these villains to keep trying?