I've got Riven on DVD, but I too have wondered why more games don't come on DVD. I guess it's because:
1. Not everyone has a DVD-ROM drive, but EVERYONE has a CD-ROM drive;
2. Retailers don't have space to "double stock" that many titles. (Though how many NEED more than one CD?)
It WOULD cut down on piracy, but if someone is gonna go thru the hassle to DL a hundred RAR files to reassemble Diablo 2 or whatever, then nothing will stop the w4r3z m0nk33z.