It isn't just about Shrapnel being an independant, smaller budget company either. Keeping the artwork simple makes the games customizable. Anyone can create a whole new universe with The War Engine and Runesword II, or create new races for Space Empires IV, without needing as masters degree in computer graphics.
$8 million in slick production may look pretty, but you're stuck with what you've got. We're still going to be playing SEIV, TWE, RSII years from now because the producers and players alike can keep the games evolving. How many "big-budget" titles can you say that about?