ccache for portage is broken since at least 2010. Because each package source gets extracted and compiled in it's own versioned directory, with each new version the cache is invalidated by the different path.

https://bugs.gentoo.org/346063

Chromium is a pyramid of components and the higher you go up, the longer the compilation of single file takes, because they depend on so many things. This makes caching difficult. A small change at the bottom can invalidate many files, especially the heavy ones at the top. The build system ninja already does caching, but it only offers limited benefits. Then there is so much activity that if you plan to do a weekly build, you can do it from scratch.

ccache is not very effective accross different versions of software as it only takes one small change in a header that's included everywhere to make the cache completely useless.