Because with the age of cloud computing and downloads, less and less people are using dvd drives. You do know you can simply download FM and play it that way, you don't always need the disc. Laptops without a dvd drive can also therefore be thinner, lighter and ultimately cheaper.
My girlfriends laptop (asus) doesn't have a dvd drive and it weighs about the same as a pencil sharpener, the battery life is also phenomenal. She's a freelance music journalist and therefore doesn't want a big bulky laptop to be lugging around everywhere like gigs and interviews With artists/bands. It all depends on need, if you definitely NEED a dvd drive either buy an external one or return the laptop for one with a dvd drive. But ask yourself the question and answer it honestly. Is it absolutely necessary to have one built in?