Factorio artstyle takes from these sources:
It's pure fantasy.
By the way, don't you find it weird how literally everything that you build is also dirty, worn, rusted and leaking?
Maybe for you and for the people who looking for own prism but not for my, i often watch reviews and in games like this are particularly checked at level of realism.
factorio is strongly technical sandbox, and speaking is realism in this game is not important is abstraction.
The concrete texture in the game at the time of me writing this (January 11th) looks too cleans, square, and shiny.
You have actually played the game yourself, right?
Just in case you didn't notice, you have landed/crashed/whatevered onto a hostile alien world. You literally start with almost nothing. This means you are all by yourself(or you have a few friends) which means there aren't billions of people to provide products to you like bleach or cleaning agents. There aren't any companies in the world to provide things to you. There isn't a government to make sure regulations are in place and things are kept in proper condition. You are all alone on this massive planets with hostile enemies that can and will destroy you and everything you have created if you aren't careful.
Lets think about this. The conveyor belts are moving ore and products around. Ore is metals and dirt clumped together, very unsanitary. The furnaces are billowing smoke and creating pollution, they're filthy and dirty. The drills are large, and create their own pollution and dirt everywhere. Assemblers are unorganized but functional, gears and levers poke out and run cycles constantly. The colors of everything aren't shiny, clean, or sanitary. The belts are grey, assemblers even with paint are rusted and worn looking. The robots are grey with yellow paint, rusted, and dirt caked. Trains and their wagons are dull, the paint is worn and torn in places, and rust, decay, and dirt are very much present. Inserters are somewhat crude looking, especially the burner ones, they move things around all the time, they pick up the dirty worn things and move them around. They're rusted and even though they have paint, it's worn in places. None of these things look new. The brick floor you can place is dark grey, it supports everything moving on top of it and certainly collects dirt and grime constantly. Not to mention the footprints and biter corpses of attacks. The walls are sloped and turrets are partially grey, but they still look worn and torn, but always functional.
The nameless protagonist controlled by you doesn't have a backstory. But even if he did, he's still alone. He can't go to the store and buy bags of concrete or bleach. Our protagonist must survive on this world with only his wits. He must create all that he can to protect himself and work on possibly getting off the planet and home. He quickly puts together inserters, drills, belts, and assemblers to make the things he needs to survive. He makes guns and clips of ammo to defend himself from hoards of enemy biters and spiters that constantly attack him from all sides. He must survive. When he is laying concrete he isn't going to be thinking to himself "I have to make perfect squares and perform constant upkeep on them to keep them shiny" no, no, no. He makes the concrete in great complicated machines that mix together iron, water, and dirty stone and then pours it quickly, unevenly, and efficiently over the ground to further his progress in this world. He needs to survive, needs to keep making things, keep designing. He himself in not even clean. Neither are cars nor tanks. The factory runs on power mostly provided by steam engines burning coal to heat water to steam as they chug away to further his machine.
One false move and he could die. He could slip and crack his head, be impaled by an inserter, or eaten alive by the vicous biters, even melted by the acid of the spitters. No where is safe, no where is clean, this isn't some government regulated factory that has thousands of workers, managers, and ceo's.
THIS. IS. FACTORIO!