Actually when working as a tech reporter full-time I asked some Google and Chromebook-manufacturer product designers this question and the answer is pretty straightforward: Chromebooks aren't meant to be powered off.
Think of the product experience. When you get the Chromebook for the first time, every single one (and from the dozen or so that I've had, I can attest to this) turns on out of the box by opening the screen. That means they are already on standby, meaning on, before they're sealed.
They have long battery life, with the shortest of any lasting 6 hours, and some lasting for as long as 14 per charge. With that kind of longevity of unlikely for users to run out of power and need to turn it off to conserve battery life. Furthermore (I love that word) people are more likely to close the lid than to shut it off manually when told to do so, and in the initial setup this is the way users are told to do it.
Lastly, it's also the manufacturer's preference. I've seen some Chromebooks, mostly from Chinese makers, that have power buttons to match to local demographic. I don't know this for sure, but it's safe to assume that most laptop users in that market are on Windows laptops that have power buttons and likely haven't used something without one, let alone a Mac, which got this design started in the first place.