Because I don't want to be a sysadmin. I want to take advantage of all the work Google has done to make ChromeOS a slick user experience, including good drivers, seamless and invisible auto-update, fast boot, integration with Google services, etc.
Owning my computer, as opposed to paying for it to be controlled by Our Benevolent Overlords.
Hence "run Debian on it". The whole reason to run a Linux distribution is that the maintainers do most the sysadmin work for you, and Debian maintainers are among the most skilled sysadmins you can find.
You have to keep your filesystems from being full, but apart from that things will Just Work as long as you stay within the Debian tooling.