Why are we so bad at drafting DBs?

Tomlin was a former DB coach and defensive guru, yet look at our HORRID track record with drafting DBs. The number in parentheses indicates the draft round.

This indicates 10 season (2009-2019) of drafting DBs.

Justin Layne (3) - TBD
Terrell Edmunds (1) - dangerously close to BUST status
Brian Allen (5) - off the team
Artie Burns (1) - BUST
Sean Davis (2) - below average to average at best
Senquez Golson (2) - never even played. Massive BUST.
Doran Grant (4) - off the team and out of the NFL. BUST.
Gerod Holliman (7) - never even played. BUST.
Shaq Richardson (5) - never even played. BUST.
Shamarko Thomas (4) - showed flashes but lacked the brains to play in the NFL.
Terry Hawthrone (5) - never even played. BUST.
Terrence Frederick (7) - never even played. BUST.
Curtis Brown (3) - never even played. BUST.
Cortez Allen (4) - actually had some decent games.
Crezdon Butler (5) - never stuck in Pittsburgh. Think the Cards picked him up for a little bit.
Keenan Lewis (3) - didn't stick in Pittsburgh but played decently with the Saints.
Joe Burnett (5) - never even played. BUST.

17 DBs drafted. 1 TBD in Layne. 11 confirm BUSTS. 5 who got some playing time but never turned into consistent starters. 0 bona fide starting NFL caliber players. Our best DBs have all come in free agency - Haden and looks like Nelson too.

Why can't a team coached by a DB guru draft a competent NFL DB?