Overall numbers: 24 QB's in, 48 OL in. So there are 5x as many offensive linemen that play each game as QB, but only 2x amount in the HOF.
Basically, you gotta be really great to get in as an OL.
This one is relatively easy to explain. Most of the time, we only hear a lineman's name on a broadcast when he commits a penalty. The rest of the game, he could play the game of his life, and they won't even mention him. The other time we hear about or see him is when he allows a sack (especially on a strip sack). Most of us on here have a more intimate knowledge of the game, but the casual fan probably couldn't name many offensive linemen. One day when I'm bored, I may do a deeper dive in terms of roster management, draft history, and cap dollar allocation to see how winning teams put together their O-lines. I might have to save that one for recovery from an injury or surgery.
As far as HOF voters go, it's hard to use a metric driven approach to assess linemen. They don't score points, they don't get yardage, and they don't have other categories to compile stats in (catches, first downs, etc.). For years, it was mostly Pro Bowl and All-Pro selections that voters had to go on. Maybe there are more detailed stats available on blocking, but I haven't seen any. If they keep stats on how many blocks someone had or % of blocks delivered vs. attempted blocks, I haven't ever seen them. Truth be told, I haven't really looked. I know places like PFF have performance scores and run blocking and pass blocking grades. But that's a fairly recent development in a game that's been played for 100+ years.
Bottom line, OL play is an integral part of winning football. While you pointed out that there are 5 guys on the field at a time, that's part of the problem. It takes 5 guys functioning as a unit to be effective, and you can't vote for a team's offensive line unit for the HOF. Skill position players accomplish a lot mostly on their own. The other issue for an offensive line is that one great lineman could be a lot less valuable playing with a bunch of scrubs at the other 4 spots. And the next time we hear a broadcaster announce that that TD was the result of PLAYER X's textbook blocking that no one else could have been able to do will be the first time.