The Celts wore helmets. And while they were certainly more 'civilized' than contemporary Germans, they're normally ranked as "barbarians" by the same observers.

Anyway, swords are kinda sucky at cutting through metal. Mail stops most blades pretty much cold, although heavy blows may still impart enough blunt trauma to make that a cold comfort (the heavy Medieval "mail-killer" war swords worked on this principle AFAIK; probably falchions too). The pointy stuff, and "mass" weapons like axes and maces, does way better. Solid angled metal surfaces are worse - plate armour, remember ? A good helmet is essentially plate spot protection for the head, and will tend to bounce most hits - and the solid ones will still lose about all their energy cutting and deforming the metal, probably never reaching the head itself in the first place nevermind now through the ubiquitous padding (layered textile and such being also pretty good a stopping blades, and kinetic energy too).

Heavy blows from heavy weapons to the head are so dangerous not so much because they were actually necessarily all that better at actually getting through the helmet, but because their sheer kinetic energy can cause concussions right through it and because the spine isn't all that strong at the neck. Dying of a broken neck despite the helmet having by and large stopped the blow sort of sucks.

Also, your assessement of the supposed uselessness of helmets against about anything but missiles is most definitely not supported by either the archeological or written record, nor practical experiments made on the subject.

And as mentioned, helmets are ultimately very cheap compared to metal body armour. Just about any decently equipped warrior could in most times afford one, but things like long swords and body armour are an entirely different story. The bit about Beowulf clearly misses this. Mail took a very long time to make and was thus very expensive; ergo, it was also very valuable, "the battlegear of noblemen", and being also very useful would obviously be prioritized quite highly for looting. Heck, victorious armies normally made a point of diligently stripping dead foes of their armour and recycling the stuff is possible - often the only way many warriors could get their hands of decent armour anyway. The Bayeux Tapestry casually presents few such scenes. And the only reason the grave-pits of Visby yielded such useful amounts of period armour is the fact the victors left the field in pursuit in the immediate aftermath of the battle, and when they returned few days later the bodies were no more in a lootable condition due to the summer heat...