It's not that Raffy is the worst 3B in the league, it's just that his bat is great enough to keep him in the lineup everyday. The other bad fielding 3b's get sent back to AAA or moved to parttime roles at least.
(Segura and JD Davis are all worse than Raffy this year per DRS. In 2021 when Devers was -13, Bohm and Dozier were as bad or worse since they played in far fewer innings.)