If I had to guess it is because if they were equivalent in quality to the armor stat-wise they would trivialize a ton of weapons in the game.
Lets be honest many many weapons are already horribly trivialized.
Revamping Silver blacksmithed weapons to have aug slots then dropping the Gemstone Inlaid Silver weapons and replacing them with Bane augs using ore + appropriate gem + whatever would go a long way towards giving Silver weapons a reason to exist. Bane damage is hugely useful at lower levels when entire zones are filled with just one mob type. You could even put the combine trivial much higher for the augs themselves and obviously they would poof when removed.Deepmetal weapons: bad ratios and no stats, but 4 aug slots depending on weapon skill (say 20 for 1hs, 21 for 2hs, 22 for 1hb, etc). Add to that a special deepmetal-level container that lets you turn other weapons into augs when combined with acidic solution or something. A 1hs weapon would turn into a 1hs aug, etc.
The augs wouldn't have the full stats of the weapon they were made from, obviously, but there'd be one custom aug for each raid-level weapon, some inheriting procs and whatnot along with some portion of stats and DMG.
Somewhere, someday, I'll make customizable items work...
Alot of ppl avoid mining this stuff and go for warpmetal or ghostmetal.