One reason I can think of: Egos. "Why should this guy get credit after having left the project years ago while I still work full-time on it and all I get is an entry in a generated contributors file? No fair!"

I mean the OP (Gabriel) hints at wanting to use it in his CV (proverbially); wouldn't anyone that made a significant contribution? Then of course, how would you define significant?

It's a can of worms and one of the ways around it is to reduce all contributors to "you are not special" status.