Mark, Doug has answered the question in his post above.
If there isn't room on each side of the centerline bunker, then the choice is lay up or hit over, or hit a dangerous shot past the bunker on either side. If the wind is up or there is OB either side, the options are reduced to lay up or carry over.
I see the need for some width - maybe 70 yards is too wide except in a generally windy environment - because there should be a reason to want to drive to either side. Maybe drive right if the pin is left or vice versa.
Viewing each side of the central hazard as a driving lane, if the entire fairway is 70 yards and the bunker is 20 yards wide, then that's only 25 yards each side, USGA width, tight driving zone.
This is based on my personal favorites: the Principal's Nose demonstrates not enough room on the right side. Left is the only play, so the fairway is effectively shifted 50 yards left. #13 Rustic Canyon, at least 70 yards of width, where's the pin, drive to the side of the fairway the pin's on 530 yards away. Otherwise you lose yardage dealing with the center bunker at the green site. #4 Talking Stick North, lots of room on both sides, where's the pin?
The fairway doesn't HAVE to be 70 yards, but the hole plays better, IMHO, if it is. You're right, if every hole has a centerline bunker or hazard (a nice creek!), the fairways can't all be wide!