Maxperson
Morkus from Orkus
Almost nothing is immune to weapons. Some are resistant, but since the game doesn't assume magic items, resistance is built into the challenge rating.You cut the rest of that paragraph. So I'll offer you a challenge.
Play a session of mid to high level play as a fighter without a single magical item. No +1 sword, no magic armor, no utility items. Further, refuse to be the target of any friendly magic either. None of your allies are under either prohibition. You are completely mundane in a world of magicals.
How well do you think you fair? Depending on the adventure, you could still pull your own weight. But you also could very easily fall behind. Monsters that are resistant or immune to your attacks. Obstacles that are difficult or impossible to overcome without mobility enhancement. Encumbrance enforced without extra dimensional storage. Etc. I wager you end up playing on the backhand, if not outright inefficient.
But sure, let's keep fighters totally mundane and remain in denial that everything special about them came from a treasure chest or a spellcaster's fingertips. Totally dependent on others, but convinced of their own independence.
I guarantee you that I would do well. Would I do as well as others? Maybe or maybe not, depending on what we encounter, but there's nothing that they could do to stop me from enjoying myself or being a hero. In fact, I would be even more of a hero than the others, since I would be overcoming more adversity to get there. If it's easy for you and there's less or no danger, you aren't being a hero or are being less of one.