That is what I said. The default out the box 5e fighter was designed under the assumption that they have supernatural power from magic items or friendly buffs.
The monsters were designed under the assumption that they would be challenged by a fighter with magic items.
Magic Items can be substituted by friendly PC spells but you would not be able to handle as many encounters per day. 5-7 instead of 6-8.
Magic items and PC magic buffs can be replaced with NPC magic buffs. But then you have DMPCs.
If you say No to all of that, you can't fight all the CR11+ demons, devils, golems, fey, undead, and dragons in the MM. I've said that from the start.