Because the cost of code review when pair/mob programming is almost zero (reviewer available next to a developer typing the code), the more likely it is to both get the feedback and to get it in a timely manner (as you type the code).
Those two traits are not available with Pull Requests. And that’s what building quality in means, which we miss out with PRs. This bites us downstream in production and increases technical debt that most teams have a problem coping with. Not to mention damaging relationship with customers, stress in the team, affecting predictability, etc.
In general, Lean terms, reducing transaction cost (cost of code review) is what enables small batches (PR size of one line of code) that enables and accelerates timely feedback (immediate and way more code improvements).