You are an outlier IMO. I respect pretty much everything you say regarding draft picks because I know you put the work in and you have a pretty good track record with Donald, Goff, Joyner and a few others.
Of course a fan can do better, because there are only so many players to pick and for one player to be great, another has to be a bust. Plus we all have an idea based on how they perform in college and the scouting combine around what round a player should be picked.
In any given draft there are probably around 20 guys who everyone knows will be a first round pick - all you have to do to be right where a GM is wrong is want a guy who ends up being good when that GM picked a bust.
But at the end of the day, teams have scouts watching a ton of these prospects in person. They (generally speaking) have more time to watch film of these players. They (generally speaking) have access to film that we do not. And they probably have connections with college coaching staffs that we do not.
In a way it's kind of like blackjack. A GM is like a player that knows how to play and follows the rules that allow for optimal success - but there are times where you can lose and lose badly anyway. Conversely, (most) fans are like the guy that sits down and hits on a 17 and sometimes has a string of luck and gets a 4. At the end of the night, the fan will walk out with more money - but the GM was the better blackjack player.