Marketing. Kept the price down(?) and made sure most people would end up by going for the ludicrously expensive somewhat basic extra's they inevitably wanted. Having the roof-mounted DVD player screen could maybe a reasonable option, but my SE doesn't even have the rear sun-blind which my 2 old 2000 75's have as standard.
True story: Back in the early 70's the original VW Golf GTI 1600 was only available in 3 colours, all of which were "special" colours in the options list at extra price! (I suppose you get one in undercoat FOC (not)). Gave the dealer something to "give away" as a special discount just for you!
David