Gentoo made a lot of sense WAY back in a pre-docker world, where you had to run the whole world on the host OS. Being able to compile it all meant you could link it all together with whatever weird combination of optional libraries you wanted. Things like getting sendmail to auth over LDAP, that nobody packaged in the base distro, become possible. And with Gentoo's tooling, you could build a coherent distro to your own unique demands.

Now though? It's way easier to have a herd of unicorn docker containers running that each handle one weird thing in isolation.