D&D isn't a computer game. Hard here is entirely subjective, you have the power to make the game as easy or hard as the players enjoy.

Your players are not optimizers or particularly tactical, and complain about the difficulty. That means you are making it too hard for them. Some players on here play in ways that would be insufficiently challenges by your games, and they would be too easy for them.

Appeal to charts doesn't help here, the reason to play D&D is that it is flexible.