So, that sounds like the kind of thing you would want if you’re making something like a drone or a router, and you have very limited resources available in the device. Compiling can be done by a the cluster you you have in the factory, not the feeble pi zero on the final product itself.
However, I can totally see why many people would want to run Gentoo at home too. It’s a pretty cool idea, and if it’s cool you might be willing to put up with the drawbacks.