The constants in the QB rating formula were set based on league averages decades ago. It would be interesting for someone to make 1955, 1965, 1975, 1985, 1995, 2005, and 2015 versions of the formulas using the averages in those years and see what ratings are under each of those versions. Probably would be a more valid way to compare across eras since you could then say stuff like "Well, Marino was x% better than a typical quarterback in his era while Starr was y% better than a typical quarterback in his era".
I've said for a long time that they need something similar to ops+ or era+ like baseball has - a passer rating that accounts for the era in which these guys played PLUS the stadium they play in. Because obviously playing in windy Chicago or frigid Foxboro is harder than playing in the comfy confines of the Super Dome.