So is all history from more than a few centuries ago. I mean, what is our entire record for the Battle of Hastings? The Bayeaux Tapestry. But people treat that as 100% solid established stuff, and then want to dismiss the very similar evidence we have about Arthurian stuff.
We have reasonable evidence to show that he was indeed a Romano-British overlord fighting against Saxon invaders who set up a court to help focus on that very goal; his feats of those and his immediate comrades were great and passed into history. It's not just those two battles, and it is fairly contemptuous of you to infer that from what I said. I used two prominent battles as examples; clearly your knowledge of this area is pretty much zero.
Junk the stone castles and the metal armour. Lancelot is a separate French legend who got added in later. Excalibur is based on Celtic legends about swords. Like all big legends, the Arthurian story picked up elements of others. But the very source of it, the British warlord who represented the last great effort of the genuine British people against the foreign invaders... history supports that.
So I will repeat- large parts of the Arthurian legend are based on fact. Not exaggeration- solid history. You, of course, rushed to an ignorant conclusion about that statement, because you have some vision of historians, instead of doing the very genuine historical and archaeological work they have done on this area, are instead just trying to falsify and lie because they are seduced by visions of Holy Grails, trying to live up to the legend. Feeble- and very, very far from the truth.
It does not matter that Camelot was a tent and not a castle. It doesn't mattter that there were no such thing as Knights at the time- or, for that matter, Kings, at least in Britain. It does not matter that Arthur predates the ideas of chivalry by centuries. because none of this stops the point under debate being true- that the Arthurian legend is not a totally fictional story, but that it is based on fact.
Whereas your statement:
"There is nothing to make us believe King Arthur existed. Nothing. Not a shred of evidence."
... is either an outright lie or an outright demonstration of ignorance. It is simply and absolutely not true. There is simply no other way to put it; there is evidence that makes people believe the basis for the King Arthur legend existed.