To write code, I effectively need to do a mini-review as I go, and type vastly slower than the AI. To review code, given that all the automated checks are out of the way, I need to consider the objectives of the code (in terms of fitting into the existing codebase, having a reasonably appropriate style, etc.) and ensure there isn't anything too far out of line. Knowing that something is "sufficiently good" doesn't entail awareness of all possibilities, because you aren't ranking the thing but measuring it.

Seniors review code because they have a much better understanding of the necessary taste and discretion, and an awareness of the larger system the code will integrate into, whereas there's only so much you can learn about coding that's relevant to most of the code you'll write (and only so fast you'll ever really be able to type it, and most of the issues can be caught by automated, deterministic systems).