You have to understand that there are groups of people who'll want to keep that physical media around. They absolutely prefer to have their content on a physical disc as opposed to digitally. It gives the feeling of actually 'owning the product' rather than leasing the digital 0's and 1's (though that's how the content providers like to view it). I'm even that way myself sometimes. If there is DLC out for a game, I'd prefer to have a GOTY version that has it on disc. Similarly, most of my music collection is ripped CDs that I bought. I don't want to have to depend on Apple's or Microsoft's servers telling me what I do or do not have the right to own.
How many times a month or year do you use your CD drive? How many of those times in the last year would plugging in an external CD drive have been a major inconvenience, or outright impossible?
Personally, I'd rather have the space in my computer used for something I actually use. I don't mind making the decision whether or not I'll toss a CD drive in my laptop bag when I go out. A USB key has replaced 80% of what I would have used a CD drive for anyway.