Football old-timers, I wonder what this indicates

In the NFL record book, there are six records listed in the "Average Passing Yards" category. The most recent holder of any of them is Greg Cook in 1969 for "Highest passing yards per attempt, rookie, season." Others are held by Sid Luckman, Otto Graham, Tommy O'Connell and Sammy Baugh. I wonder why, as passing became more and more dominant in pro football, that the records in this particular category are all at least 47 years old. All of them. Maybe, as teams passed more, they became more cautious in their passing? I don't know.