But those are the very last bosses and should be hard. I can guarantee you, fans of the franchise would be furious and/or devastated if you could just slap them around on your first try. In fact, the punishing nature (and the huge rush after overcoming a challenge) of this game is one of the cornerstones of its popularity. I can guarantee you that adding difficulty sliders would upset a lot more people than leaving them out would
Also, with the open world nature of this game its already challenging enough to get the balance right, without the added complexity of difficulty sliders. If you are truly stuck then you have loads of options. You can farm to level up, you can upgrade your weapons, you can respecc, you can upgrade spirit ashes and if all else fails, you can summon another player with an OP build who just demolishes the boss while you stand in the back and take sips from your flasks
Lastly, if you made it all the way to the last bosses, but still find the difficulty/gameplay unappealing then you're just a stubborn fool imo.