Because they're old.
That is the actual reason. Old things are powerful, just because they are old.
For example, IIRC in Kara no Kyoukai, there was a 200+ year old katana that destroyed bounded fields when drawn. It hadn't been enchanted or anything, it had just been around for a while, and that's all you need to accumulate mystery.
There's also the 'Nasu history is weird, and most of the time, the heroes actually had that power in life' factor.
But then you get cavemen with superpowers!
And the most powerful weapon ever should be a random stone axe which survived to the current age.