I don't disagree with you at all, but I'm not sure I've been a part of an organization that does it right. Beyond the stuff like style, conventions, and obvious bugs/problems, it's challenging to provide meaningful insight to someone that has many more hours of experience in a particular area of a codebase more than you do. I know I personally hold back from calling out things that I find to be subjective -- even if I think I would have done it differently -- simply because I'm often unsure about the things I don't know.
Plus as a peer reviewer, you're often in the same part of the sprint cycle as they are - meaning your mind is already juggling a lot of things about your own code. It's tough to "come up for air" and then immediately dive into someone else's code changes with enough mental capacity to provide a truly useful review.
It's easy to slip into the rubber stamping mentality when you generally trust the ability of a particular contributor.