Why doesn't Gentoo have more distros based on him?

There are hundreds of Gentoo-based distos out there.

None of them have finished compiling yet.

Why are you defining the gender of a Linux distro lmao

It's pointless to make a derivative distro if the base is already perfect.

I guess kinda the same reason why there are not many distros based on Slackware.

Because Genthree Genfour Genfive Gensix, etc. would get confused with other products on a Google search.

I think it's somewhere around 15 derivates for Gentoo. 1 2

It's about the same number as lots of other distros?! I don't see any substancial difference there. Except when you compare it to something like Debian. There are some hugely popular distros.

it's very customizable.

People make new distros based on existing distros to alter the software stack in the original distro. Gentoo allows you to use essentially whatever software stack you feel like, it's less of a traditional distro and more of a general-purpose Linux package manager and dependency resolver. It's not very opinionated at all.