Wanting to have control over config via use flags for your system doesn't mean that there aren't packages were you don't really need that. Like if you only use Libre Office a couple times per year on your aging laptop, do you really care enough about the exact USE config to justify compiling it yourself? Even more so if you need it on short notice. Or if you only use Chromium/whatever to check that your website works with that browser but don't actually use it yourself, why bother compiling it.

IIRC there used to be a Gentoo fork (forgot the name) that extended this concept to all packages, so if you used default USE flags you did not need to compile things yourself.