Why is there such a big disparity between the price of cheap (generic) flour vs. name brand (such as King Arthur)?

Look at what you did.
You said cheap brand, and good brand.
More expensive is so automatically thought of as better, often pricing is chosen not by actual value, but what they think they can get away with.

King Arthur is a top quality (fewer rat dropping, cockroach legs and better quality grain) flour milling operation. Higher cost in the eye of the consumer is okay for higher quality.

The cheap flour isn’t re-wrapped by King Arthur for a higher mark-up.

King Arthur brand is still owned by the British government; nominally in the name of Queen Elizabeth, as she is successor (750+ years later) to King Arthur.

The Windsor family still receives royalties from the sale of flour, just as they receive rents from people owning houses in London and across Great Britain. It is this sort of business-government-monarchy setup that has kept the British throne as wealthy as has been, and continues to be.

In the 1500s and up to more industrial times, the royal family grew their own grain and milled it – in other words, it was a vertically integrated business. No doubt slaves and unskilled workers added to the profitability of the flour business.

However in the 1800s and up to now, it is far easier (and more lucrative) to industrialize the manufacture of flour, to royal standards, of course, than to grow their own.

The key point to remember is that Queen Elizabeth and her kin earn a little each time you buy a bag of King Arthur flour.

@elbanditoroso It is employee owned and located in Vermont, not Norwich – - East Anglia, England

NOT SO see King Rrthur Flour

NOTE: OP said, “such as” we are not speaking of any specific brands here.

Oh @Patty_Melt you can decide what is a high priced flour!

So why are you arguing about brands of flour?

I commented on the brand included by the OP.

@elbanditoroso said King Arthur was run by the Queen of England. No tildes in sight !

I’m not arguing about anything. OP asked for a comparison, but between prices, not brands.
I guess you can discuss brand origins if you want, this isn’t in general, but it doesn’t answer the question. I just pointed that out is all.

Because it’s branded.

A branded turd is still a turd, but it’s shinier and smells like vanilla. You basically pay for the nice smell and tinsels.

Typically, a more expensive flour (King Arthur, Montana Sapphire, Bob’s Red Mill, Hodgson’s) is a higher quality flour. This link is to a website called Weekend Bakery. The experiment they conducted does a much better job of showing the differences in using lower quality and higher quality flour.