Thats exactly what i'm saying, have you read the fluff? The Leman Russ is a tractor with a big gun bolted on top, the Mechanicum don't make new things and unless they have found the plans for something, they struggle to figure it out. They lost all knowlegde and have been trying to regain it for over 10,000 years, the idea of inventing new things is heretical to the Imperium.