Except when they kinda do.
For example consider Alfred Morris's preseason performance.
Or consider the guy who threw the 2nd most passing yards this pre-season. He's sporting a higher QB rating than Andrew Luck and currently is projected to be next season's Eagles' starting QB.
The reason that rookie QB finished 2nd in the preseason was because the leader, a rookie named Kirk Cousins, threw for a whopping 560 yards.
Preseason stats don't mean a thing...except when they kinda do.
Preseason Bears game highlights: https://www.youtube.com/watch?v=kGtz_sDLUfE
Regular season Ravens game highlights: http://www.nfl.com/videos/nfl-cant-miss-plays/0ap2000000107958/WK-14-Can-t-Miss-Play-Cousins-forces-OT