Because a code review is not to pat each other on the back, it is to inspect and report issues. It is already costly enough without going off topic.

As said, if you want to praise then you are free to do it offline.

This is a professional procedure in a professional setting, not warm words from an encouraging teacher at school... I don't want to have to go through comments that do not add any value to the exercise of finding issues, and I have never seen people leave such comments in 20 years.