There are a lot of reasons, as outlined above.
Removing the drive reduces weight and thickness, so Apple can advertise how thin and light their machines are.
It also reduces cost a little bit. Apple is already known for having expensive computers (you can argue all day about whether they are "worth it", but the bottom line is they do cost more than a "typical" PC).
(This helps twofold, one by reducing the physical cost of the machine due to fewer parts, two, adding to the perceived value of the machine because thinner and lighter PCs are supposed to be more expensive.)
The space taken up by the optical drive can be replaced with a larger battery, for longer life.
Optical drives are moving parts and are prone to failure. The DVD burner on my 2007-era MacBook Pro works only intermittently now. Not including it adds to perceived reliability since the only moving part would then be the hard drive -- and maybe not even that, with today's SSDs.
USB sticks today have capacities in the 32-64 GB range, so they are convenient for large file transfers. This simply wasn't the case 5 years ago.
More and more people are moving to digital downloads or streaming, e.g. iTunes, Netflix, rather than buying CDs and DVDs or Blu-Rays.
Speaking of Blu-Ray, Apple seems hell-bent on NOT including them with their machines. So rather than coming up with excuses, it is easier for them to simply say (and spin) "we don't need ANY optical drive!"
The bottom line is that we simply don't use CDs and DVDs as much as we used to. Yes, I still use them occasionally, but I bought a cheap USB drive. Actually, I bought the cheap USB drive when my internal DVD burner started to fail, so it was kind of necessary anyway.
Even years ago, companies acknowledged that you did not always need CD/DVD drives. Back in 2001, my Dell laptop had two swappable bays which you could configure as one battery and one DVD drive, or two batteries.