Optical media is so 90's early 2000's. My entire Steam library is digital, when I built my new PC in December, I didn't even bother putting my Blu-ray drive in the free drive bay. Windows was installed from a SD card, the first time I've used such a device to install an OS.
In my country - the United Kingdom - there are no physical PC games in stores, at least not any new ones. The last game I purchased on disc was Civilisation V.
There is one major factor you've all missed about why PC games have gone digital, and that is the size of modern games and Blu-ray disc, or rather the lack of PC BD disc support and BD-Drives on PC's. Unlike PC CD-ROM and then DVD PC games discs, Blu-ray never made the transition to PC as the medium for shipping PC games. Without the larger disc capacity that BD disc provides, PC game publishers would either have to distribute their games on multiple DVD's or provide one or two discs and the rest of the game as a download.
Imagine how many DVD discs a game like the 113GB Gears of War 4 would need for a full install on DVD discs?