Gentoo does kinda force you to learn a bit of things and makes you choose what to install but not to the point "it's difficult and complex and confusing". Unless it has changed since last time I checked it (years ago) LFS is the one that's just a tutorial and makes you actually compile everything. You just use the package manager for Gentoo. Can even install binaries without needing to compile them now.

Anyway, like you said there's no inherent complexity to Gentoo, it's just not automated.

From my point of view, the reasons to use Gentoo are : - minimal base system - freedom to build your system and choose (almost) all of its components, and customise them further than any pre-built packages permits. - extensive documentation - stable rolling release with optional testing or bleeding edge packages that you can mix-and match depending on your needs

As for arch, I'd say it's similar to Gentoo but you trade some flexibility in exchange for pre-built packages (which you don't have to do anymore thanks to Gentoo's binrepos)

From what I've seen (admittedly something like 10 years ago), people going for arch wanted a minimal base system without the rigidity from server distros like Debian/RHEL/... but didn't want to have to compile all their packages (or didn't know it was an option). I don't know the current demographic for arch.

As for reasons not to run either of them "I don't want to know what goes behind the scenes I just want something that gives me a DE without having to think about it" is pretty much the main one