IMHO because it's not something trivial to build so you would also need to build the stuff to build it. To build complex (and huge) machinery you need specialized factories with lots of other big and complex machinery, you cannot just create a factory to build n=1 objects it's not economically viable.

Though, perhaps they meant "assemble".

A lot of other big things, like wind turbines, are shipped in smaller pieces and assembled onsite. At least according to a family member of mine who works on them, you even get slightly different designs that are specifically tailored to the local highway transport regulations in different countries.

That said, I think in general you're right that the story is that it's just cheaper to do it this way. I just think that assuming you'd need to build actual factories is a bit drastic. But perhaps even getting the equipment needed for final assembly onsite is prohibitively expensive compared to just transporting the fully-assembled equipment.

Well if they mean assemble instead of build the things change a bit, but anyway even when talking about big and complex things there are big and complex things that can be "easily" split into pieces and assembled at a later time and other things that cannot be split so easily so much more resources are needed to assemble the thing in its final form.

Just guesses it's not something I'm into.

The short answer is probably that the government is throwing so much money at this, and it’s usually an uncapped direct government expense to do offsite utility work like this for these kinds of major projects. Compare to making it an “onsite expense” which will have a dollar cap.

Cost and quality. I worked on a new chemical plant in Singapore. Everything was fabricated as modules in yards all around the world. A lot of work can be done in lower cost countries, like building pipe racks in the Philippines. Ship all the modules to site then everything is bolted and welded together, wired up. The only stuff that's really done at site (besides integration) is the underground utilities, foundation work, and then commissioning.