I firmly believe 20 minutes is the optimal amount of time to meet, introduce a problem and evidence, and make a decision that allows all parties to move forward aligned.

I suspect office work defaulted to 30 minute calls because of TV/Radio broadcasts in the early 20th century, but I haven't researched this theory further.

I think humans just prefer halves and quarters over thirds. So, you either get a full hour, a half hour, or a 15 minute meeting.