Simple. Because these "features" are not needed and raise the cost of the system.
Nintendo has stated they are making something known as a "gaming console". It plays GAMES. Not DVD movies. If you want that, you can buy a DVD player (or another version of the GC released by Panasonic, I believe). Either way, this allows them to charge less and focus on one thing..GAMES.
The Xbox will allow you to play DVDs, if you buy the special remote (or something like that). But, MS decided not to focus on the DVD playback, because once again, its a game console. You buy it to play games. Not movies!
Sony is trying to make the PS2 a general entertainment unit. Music, movies and games. Problem is, DVD playback isn\'t the greatest and in Japan people was buyin the PS2 just to pay DVDs (a ps2 was cheaper then a DVD player).
I personally wish DVD playback was never brought into the PS2, but that is just me. Then again- I am bitter that it doesn\'t play my HellRaiser 2: Hellbound..