Others are right - it's mostly SDC and IROPS. And F passengers can confirm right into any available seat.

A big reason people pony up for F is to gain this flexibility, so a larger proportion of people take advantage of it than you might think...

I agree it can be demoralizing as a planned-ahead coach passenger - especially when you picked a flight with desirable equipment and now others want to snipe the seats. But it's how the game works.

If you can't beat em, join em; consider buying F seats, or using a RUC when the upgrade really matters to you or when you want the SDC flexibility for yourself.