You can buy the Windows 8 Media Centre Pack for £6.99 if you want it (they did give this away free in the very early days as a special offer).
Believe it has to do with the fact that MS have to pay licenses for every copy for the codecs. It has come in and out of windows over the years - a number of versions you had to buy 3rd party products to do DVD playback.
no as ian72 said, its a move microsoft made to save money.
In XP, vista and win7 the dvd codecs were preinstalled, but because of that microsoft had to pay for a licence for every cop of windows sold. Now they dont because the media pack isnt included by default.