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.