Cognitively, these are very different tasks. With the former, we actively drive technical decisions (decide on architecture, implementation details, even naming). The latter offers all these decisions made, and we first need to untangle them all before we can scrutinize the details.
What's more, often AI-generated code results in bigger PRs, which again adds to the cognitive load.
And some developers fall into a rabbit hole of starting another thing while they wait for their agent to produce the code. Adding context switching to an already taxing challenge basically fries brains. There's no way such a code review to consistently catch the issues.
I see how development teams define health routines around working with generated code. Especially around limiting context switching. But also retaking tasks to be made by hand.