Earlier quoted context omitted.
Probably because the Gentoo ecosystem (package tree, package managers) is build for source packages and not for binary packages.
Portage can work with binary packages just fine: https://wiki.gentoo.org/wiki/Binary_package_guide ChromeOS/ChromiumOS uses portage for its build system, to produce a signed, read-only image that the OS can boot from. CoreOS uses the same tooling FWIW: http://www.chromium.org/chromium-os/build This is why you can’t just install random packages onto the parent OS. Any additional software needs to run in some overlay f…
Package managers which are build for source packages (e.g. ebuilds/exheres) have a much more complicated logic running in the background, come up with much harder to solve problems and are a lot slower (orders of magnitude).
The only aspect where this setup is superior than e.g. arch + pacman is when it comes to using source packages on the same system. So I guess that is nothing you would like to use on a Chromebook anyway. Therefore, using a Debian container is a reasonable choice, in my opinion.