> vividly recall something that happens on the order of once a year as "oh yeah that's common".

I mean, think of how many times a typical person has sex in their life. Hopefully they and their partner aren't getting pregnant more than roughly once per year. But somebody getting pregnant after having sex is reasonably defined as common. Certainly common enough that it's something you would consider and take precautions to prevent if you didn't want it to happen.

In ranked chess games, underpromotion happens about 1 in 1000ish games. I imagine it would be more common in high level unranked play. If you play one chess game per day, that's once every 3 years on average. It's not frequent, but I'd describe that as common.