Here's Mars. You know, the hardest working planet in technology and technology accessories.
See all that red stuff? That's all the not-factory of the planet. There's nothing being made there.
You're STILL forgetting that for each land raider, on top of the assembly line that puts it all together, you need factories that cast the hulls, factories that assemble the track links, factories that cast the track links, factories that make the pinks that connect the links, factories that manufacture the engines, factories that assemble the parts of the engine, factories that make the guns, factories that make the power plants for the guns, factories that assemble the computers, factories that make the chips that go into the computers, factories that program the chips, factories that make hinges for the ram doors, processing plants that produce the raw materials that you make the hulls, chips, engines, guns, etc., and on top of all that, you need mines to dig up all those resources to produce the ore for the processing plants.
Then there's all the warehouses that store all these materials, conveyor systems to transport materials across the planet, facilities to research new technology, facilities to test new technologies, archives to keep all the info, housing for the hierarchy of the Mechanicum, etc.
Then you need garrisons for the planetary defense force, because you're basically the heart of the Imperium's land raider manufacturing, there's bound to be a sizable force protecting such a valuable asset. That force needs to be provided with guns, armour and equipment. And living space, training space and room for all their tanks and planes.
Then you have billions of workers, who need housing, food and goods. For that you need farms, processing plants, factories, etc. And those places need resources.
For each assembly line shitting out a land raider whenever it can, you have VAAAAAST infrastructure set up to make that one assembly line work.