5 years ago, an optical drive was a necessity. CD/DVD was still the main distribution method for software -- for Apple as well, in fact they were rather late to the e-distribution party. But today, when even Microsoft has started selling downloadable Windows and Office, it's a different story. Every single one of the dozen or so software companies I rely on for my business (Apple, Microsoft, Adobe, Propellerhead etc) have shifted to downloadable installations.
That leaves music CDs and movie DVDs. As for music CDs, the MP3 music piracy thing has been going on for 15 years now, iPods and iTunes Store have been around for 10 years, and for the last few years the norm here in Europe has been to get your music from Spotify for free. I don't know a single person who buys and listens to CDs. I ripped mine long ago but I still have them on a shelf in my living room and when people stop by, they're like "You have CDs? Wow, didn't know you were so retro".
Movie DVDs... how are they relevant on an iMac? A Mac Mini hooked up to a TV maybe, or a laptop for watching movies on the road, but a desktop computer isn't exactly a home theater, is it? (Unless you live in a dorm room the size of a closet). There's a reason why theaters don't have office chairs.
Optical drive isn't completely dead, but it's a legacy device, and Apple was never big on legacy support -- especially when it comes to the iMac. The original iMac, a desktop machine with no floppy or 9-pin/25-pin, set the tone for what you should expect from that model.