Why are linebackers (Veterans and Draft prospects) never really good in coverage

While I’d say I’m not a noob it’s been a question on my mind. While looking through free agency and the league I’d say 1/12 linebackers are fine in coverage which is weird. It’s worse in the draft too. There’s really no “good” coverage linebackers and I don’t remember the last time I’ve scouted one. Is it a developmental issue, or just focusing on other things?