We used to do 6 monthly reviews but scrapped them in favour of more regular meetings every 4 - 8 weeks, largely for the reasons you've stated. That being said, in my experience reviews are one of those tasks that take up as much time as you give them. If you only give yourself a month to gather feedback and prepare them you'll find a way to get it done, and I always wanted to work off the most current information available when meeting with members of my teams, hence doing the prep as late as possible.

As I said, GitHub is not a large company at around 700 employees, so I'm not really sure what their excuse is. The largest company I've worked for, as a contractor this time, was around 400k employees, and they seem to have much better processes around this with regular one-to-ones in the place of annual reviews, at least for permanent tech staff.