As a Clojure developer focused on creating correct/working software, I find the "Simple"-ness of my tooling's internal architecture to be a lower priority than it not leaking additional cognitive load onto my dev experience.
It may be the case that lein's internals are a blob of complexity, without boundaries defined in the "right" places. Maybe Clojure Tools' 3 or 4 components have it correct (though I'm skeptical of that, since there's overlap between them -- even moreso now with tools.build).
Conversely, it may be that build tooling is by its nature a messy affair (dealing with dependencies, resources on the filesystem/network, interaction with tools, tests, etc.), and that making those problems go away to the extent technically possible does your community of developers the greatest service. That would explain why talk of CLI and deps invariably has pushback and lein advocacy.
I wouldn't know if lein is internally complex because I've never had to go digging in it beyond the documentation project.clj file, even to do rather complex build definitions. By comparison, I've easily spent man-weeks on getting to the same place with deps.edn, and I would still find even defining the roles of its components difficult if pressed. I also worry that if we hit new Clojure devs with a wall of tooling issues like this, they won't stick around, even if we tell them the underlying theory of their tooling is more sound.