I make up a lot of my own lore (so I can ignore the dumb stuff and focus on what's cool), so for me it's more fun to imagine orks have an idea of how machines should work, but most of them aren't smart enough to make it actually function without it breaking. Thus things are held together by duct tape, copious amounts of elmer's glue, and mostly latent psionics. An attempt was made, but no ork, engine no work.