If I can install a refinery on my Type-7, why won't that scale to a dredger?
Your refinery makes pure versions of a given ore. It doesn't make Steel. It doesn't make whatever composite hulls are made out of. That's a process that requires a much larger and controlled set of systems to produce. And it would likely not be machinery that can be used to say create plastics, glasses, or other polymers ..much less even other alloys.
Most of the bulk of the dredger is involved in breaking down, storing recycled material such as those processed alloys and polymers and such and being able to manufacture them back into usable products.
The equipment and such needed to skip the recycling would negate the entire purpose of why the dredgers were made. Because it would have to have a means for creating most if not all of the necessary things from raw materials.. . not just a handful of specific things like how most refinery stations and settlements work.
And if there are thousands of people on one, what do they do all day? Do they sit in their compartments all day and read the forums or play "Thermonuclear War" or perhaps "Chess" or maybe even "Oregon Trail"?
I'd imagine the crew of a dredger do nothing all day every day but replace broken parts with new parts considering the size and age of them.
elite dangerous: tales from the frontier
It seems to me you're "thinking too hard about it".
Have you considered that the Elite Dangerous "Universe" might not be the one for you?
nope. Have you considered that the elite dangerous "universe" needs those like me to witness it's crumbling ...to stand on the sidelines unable to stop conflicting lore so that the future may avoid repeating such a fate again?