Mixed-level pairs (senior/junior), for example, are more about mentoring than reviewing. Those sessions do not qualify for "more than one pair of eyes".

Excited (or maybe even stubborn) developers can often win their pairs by exhaustion, leading to "whatever you want" low effort contributions.

Pairs tend to under-document. They share an understanding they developed during the pairing session and forget to add important information or details to the PR or documentation channels.

I'm glad it has been working for you. Maybe you work in a stellar team that doesn't have those issues. However, there are many scenarios that benefit a lot from an independent reviewer.