Because of how hard to get they are. It's been over a year since land of Iron dropped and kraken and dark Weasel became world bosses. Yet how many end game weapons do you know of in-game? Yamazaru is the only LOI sword in-game, hammer is gone (And there was only ever one dropped), and there are like 6 dark scythes. The amount of people that have these is extremely low and if an item has a literal 1:1000 chance of dropping it sure had better he worth the effort. The people that complain abput their strength are the ones too lazy to put in the effort to get one themselves or too broke to afford one of the very few that have been dropped.
The rarity offsets the power. Not everybody in Middle Earth can possess the one ring.