There are many reasons. First a manager is not a peer but brings in a sense of authority into the mix so the discussions will not be honest. Manager's inputs have a sense of finality and people will hesitate to comment or override them even when they are questionable.
There are human elements too. Even if someone has honest inputs, any (monetary or otherwise) rewards or lack of them will be attributed to those inputs (or lack of them).
Overall, it just encourages bad behaviours among the team and invites trouble.
These should not happen in an ideal world but as we are dealing with people things will be far from ideal.
Even when done with good intentions, managers being involved in code/reviews almost always ends up being net negative for the team.