Most of the time a dev isn't thinking "I'd better make this selectable," because they simply aren't going to think of all the reasons someone might select text. They'll think of the obvious-to-them cases, and leave all the other text as unselectable.
It's like if you made spoiler 50/50. Do you want to see this without clicking?
I think we really overlook the thought that had gone into making js, and other open tech.
I truly think my random clicking on paragraphs that I'm approaching isn't just ADHD but a way of helping my eyes stay on track. It's very disconcerting when the text doesn't select. (As, clearly, all these comments noticed as well -- why were we all trying to select the text? To check if it's Flash-ish, of course, but also we just like things to grab onto while we read.)
Or even translating...
Try putting their demo website in Google Translate or even try to select the text on an email in their demo webmail app to copy paste it in google translate ....