I think the bigger answer to why they don't include an optical drive is that it's seeing less and less use and is a somewhat unreliable mechanical component. Apple did continue selling the 13" unibody MBP until either late last year or early this year(time runs together)-it was a dated model, having been last revised in 2012(said with the caveat that a 2012 MBP is still my main computer) but did retain an optical drive along with ethernet and Firewire.
For those who still require an optical drive, Apple does sell one as an add-on.
As is often the case, Apple has been ahead of the curve on ditching what they see as legacy technology. Everyone went into an uproar over the first iMac not having a floppy drive in 1998, and plenty of folks either bought a USB floppy or LS-120 drive. By 2000, Apple had ditched built-in floppies completely, and no one even thinks about it now. The first "modern era" computer without an optical drive was the MacBook Air in 2008, but they went full scale in 2012 and eliminated them from almost everything. In 2008, the USB Superdrive was an almost automatic purchase with a MacBook Air. I'd be interested in knowing how many they sell these days.
As for reinstalling the OS-10.6.3(Snow Leopard) was the last version of OS X/macOS distributed on physical media. Everything since then has been distributed through the App Store. Lion and Mountain Lion were paid downloads, while everything since has been free. Since I often do support for other folks, I tend to keep the installers handy-a quick check shows that I have everything except Yosemite on hand(and I'm not rushing to the app store to fix that-Yosemite has probably been my least favorite release).
And, like folks said above, I just make USB installers when I need them. They only take a few minutes to make, and installing from one is a LOT faster than installing from a DVD. A quick check shows that the 10.11 is 6gb and 10.12 is 5gb-if you wanted to burn that you're in the realm of a DL-DVD, which is both expensive and can be fussy to burn correctly.
BTW, the original MBA even shipped with a USB installer pre-made.
Internet recover also does work very well-albeit somewhat slowly-if you are "deserted" without any other Macs around and don't have a back-up.