That's a good point, but I think adherence to an internally formulated consistency is easier than adherence to an external and by extension infinitely more vast pre-existing consistency.I don't think historical accuracy and accuracy within a fully imagined world is all that different. For an author in a series, they have to cross reference everything that came before it. Just like a historical mistake, readers will call out an author for discrepancies in their own world.
I think it's easier to write about the world in your head than write about a fully realized time/place and remain true to an accurate degree.
Your post does raise an interesting question though - does a lack of internal (or external) consistency show a lack of appreciation on the authors part if the reader knows their world better than they do?