The Raiders have been to five Super Bowls and the QBs on those teams were Daryl Lamonica, Ken Stabler, Jim Plunkett and Rich Gannon. Lamonica was drafted in the 12th round of the NFL draft and in the 24th round of the AFL draft. The Mad Bomber was acquired by the Raiders in a trade. Ken Stabler was drafted in the second round of the NFL draft by the Raiders. Jim Plunkett was the 1st pick overall in the NFL draft by the Patriots. Plunkett was signed by the Raiders after his outright release by the 49ers. Rich Gannon was a 4th round selection by Vikings and he was eventually signed by the Raiders as a free agent. So of the four quarterbacks who led the team to five Super Bowls, one was drafted by the team and he was drafted in the second round.
The Raiders have drafted 4 QBs in the first round: Jamarcus Russell, Todd Marinovich, Marc Wilson and Eldridge Dickey. Dickey was immediately converted to a WR so it's really inaccurate to call him a QB. None of the other three ever started a playoff game for the Raiders. Wilson, however, was a backup on the 1980 and 1983 Super Bowl winning teams.
The point is that Super Bowl QBs come from lots of places. Just think Tom Brady (6th rounder), Drew Brees (second round), and Kurt Warner (undrafted). Certainly, many successful QBs are drafted in the first round but there are no guarantees. And a bad choice of a QB in the first round sets the franchise back for years.