It was just a trend I noticed, or maybe a difference in cultures. I notice my coworkers sometimes spending 15-20 minutes going over my code sometimes in phab. While at my past place they didn't dive deep into the code. People also don't feel as comfortable bringing up pedantic crap in person as much as they do over a web interface as you said.
I think with OTS your going at the pace of the presenter, while with an async code diff, the reviewer is going at their own pace, can hop back and forth in between things and so on. It's also hard to keep track of things you have to change with an OTS, while with async code reviews the request list is right there already.
Sometimes we do group or individual OTS style CRs (new term for me, "OTS"), but with a twist. The reviewer walks through and describes the code, not the one who wrote it.
I think with OTS your going at the pace of the presenter, while with an async code diff, the reviewer is going at their own pace, can hop back and forth in between things and so on. It's also hard to keep track of things you have to change with an OTS, while with async code reviews the request list is right there already.