But then you are working on half as many things.
Works goes faster when you work with another person even if for the fact it is harder to procrastinate.
And problems are always easier and cheaper to solve the earlier in the process they are caught.
Then there is the simple fact that a well executed coding review will take significant portion of the time it took to make the change. Most coding reviews are really only cheaper because the reviewer is just skimming the code to see for obvious faults. So it is not really apples to apples comparison.